1 Introduction

As a special class of nonlinear optimization problems, quadratic optimization has received extensive attention by many researchers and used in a wide range of fields, such as game theory, signal processing, and portfolio optimization. In recent years, there have been various excellent works on the investigation of quadratic optimization problems from different perspectives, see [1, 2, 15, 16, 23, 33, 34, 36] and the references therein.

Recently, quadratic optimization problems with uncertain data have attracted extensive interest of many researchers due to their applications in different fields of mathematics, engineering, and economics. Robust optimization approach [4, 6, 17] is a powerful methodology for dealing with quadratic optimization problems with uncertain data. For example, by using the robust optimization methodology, second-order cone programming reformulation problems for convex quadratic optimization problems on different kinds of uncertain sets are obtained in [26]. Exact second-order cone programming relaxations are established in [19] for non-convex minimax separable quadratic optimization problems with multiple separable quadratic constraints. In [24], exact copositive optimization reformulations are obtained for robust quadratic optimization problems with uncertain parameters containing both continuous and integer components. In [13], a deterministic approach is given to examine robust optimality conditions and find robust efficient solutions of convex quadratic multiobjective optimization problems with uncertain data. By virtue of a new robust type characteristic cone constraint qualification, second-order conic programming dual of robust convex quadratic optimization problems on polytopic and norm uncertain sets is considered in [37].

Note that all of the above papers are concentrated on the investigation of static (single-stage) robust optimization models, containing only “here and now” decisions variables. This means that before obtaining complete information about uncertain parameters, we must now determine their values [9, 10, 21, 27,28,29, 31]. However, many dynamic decision models contain not only “here-and-now” (first-stage) decision variables, but also “wait-and-see” (second-stage) decision variables which are assigned numerical values after some of the uncertain parameters are known. Adjustable robust optimization, introduced in [3], is an important deterministic methodology to deal with optimization problems involving both “here-and-now” and “wait-and-see” decision variables. Although there are few papers in the literature devoted to adjustable robust quadratic optimization problems, see, for example, [7, 8, 11, 12, 32, 35], adjustable robust quadratic optimization problems have received far less attention than others. This is a main motivation for the investigation of adjustable robust quadratic optimization problems in this paper.

Motivated by the works reported in [7, 12], this paper will establish exact SDP reformulations for a class of adjustable robust quadratic optimization problems with affine decision rules on spectrahedral uncertain sets [25, 30]. More precisely, by using a special semidefinite representation of the non-negativity of separable non-convex quadratic functions on box uncertain sets, we show that this adjustable robust quadratic optimization problem admits an exact SDP reformulation problem in the sense that they share the same optimal values and their optimal solutions are one-to-one correspondence. The result obtained provides us with a way to investigate adjustable robust quadratic optimization problems by considering the corresponding SDP reformulation problems. For ellipsoidal, polytopic, and box uncertain sets, we also establish exact SDP reformulations for adjustable robust quadratic optimization problems. We show that our results cover as special cases of some optimization problems considered in the recent literature [12, 32]. Furthermore, as an application, the proposed approach is applied to investigate exact SDP reformulations for fractionally adjustable robust quadratic optimization problems on spectrahedral uncertain sets.

The rest of the paper is organized as follows. In Section 2, we recall some basic notions and introduce an adjustable robust quadratic optimization problem. In Section 3, we establish exact SDP reformulations for adjustable robust quadratic optimization problems on spectrahedral uncertain sets. In Section 4, we consider exact SDP reformulations for fractionally adjustable robust quadratic optimization problems.

2 Preliminaries and Auxiliary Results

Unless otherwise specified, \(\mathbb {R}^{n}\) signifies the n-dimensional Euclidean space equipped with the usual Euclidean norm \(\Vert \cdot \Vert \). The inner product in \(\mathbb {R}^n\) is defined by \(\langle x,y\rangle :=x^{\top }y\), for all \(x,y\in \mathbb {R}^n\). The zero vector of \(\mathbb {R}^n\) is denoted by \(0_n\). The non-negative orthant of \(\mathbb {R}^{n}\) is denoted by \(\mathbb {R}^n_+:=\left\{ (x_1,\dots ,x_n)\in \mathbb {R}^n~|~x_i\ge 0,i=1,\dots ,n\right\} \). The space of all symmetric \(n\times n\) matrices is denoted by \(\mathbb {S}^{n}\). \(M\in \mathbb {S}^{n}\) is said to be a positive semidefinite matrix, denoted by \(M\succeq 0,\) iff \(x^{\top }Mx\ge 0,\) \( \forall x\in \mathbb {R}^n.\) Moreover, \(M\in \mathbb {S}^{n}\) is said to be a positive definite matrix, denoted by \(M\succ 0,\) iff \(x^{\top }Mx>0,\) \(\forall x\in \mathbb {R}^n\setminus \{0_n\}.\) The symbol \(I_n\) stands for the \(n\times n\) identity matrix, and the symbol \(\text{ O}_{n \times n}\in \mathbb {R}^{n\times n}\) stands for the \(n\times n\) matrix of all zeros. The matrix \(D:= \text{ diag }(r_1,\dots ,r_n)\in \mathbb {R}^{n\times n}\) stands for the diagonal matrix with \(r_i\in \mathbb {R},\) \(i=1,\dots ,n.\)

In what follows, let \(Q_0\in \mathbb {S}^{n},\) \(q_0\in \mathbb {R}^n\), \(\xi _0\in \mathbb {R},\) \(Q_i:=diag (a_1^i,\dots ,a_n^i)\in \mathbb {R}^{n\times n} \), \(q_i:=(q^1_i,\dots ,q^n_i)\in \mathbb {R}^n \), \(q^i_k:=(q_{k}^{i1},\dots ,q_{k}^{in})\in \mathbb {R}^n,\) \(\xi _i\in \mathbb {R}\) and \(\xi ^i_k\in \mathbb {R},\) \(k=1,\dots ,p,\) \(i=1,\dots ,l.\) In this paper, we consider the following uncertain quadratic optimization problem

$$\begin{aligned} \mathrm{{(\text{ UP})}}~~~~~~~~~~~~~~~\left\{ \begin{array}{ll} \mathop {\text{ inf }}\limits _{x\in \mathbb {R}^n} ~~f_0(x)\\ ~s.t.~~~f_i(x,u^i)\le 0, ~i=1,\dots ,l, \end{array} \right. \end{aligned}$$

where \(f_0(x):=x^{\top }Q_0x+q_0^{\top }x+\xi _0\) and \(f_i(x,u^i):=x^{\top }Q_ix+q_i^{\top }x+\xi _i+\sum _{k=1}^p u^i_k\left( (q^i_k)^{\top }x+\xi ^i_k\right) ,\) \(i=1,\dots ,l\). \(u^i:=(u_1^i,\dots ,u_p^i)\in \mathbb {R}^p\), \(i=1,\dots ,l\), are uncertain parameters, belonging to the following spectrahedral uncertain sets [25, 30]

$$\begin{aligned} \mathcal {U}^i:=\left\{ u^i:=(u_1^i,\dots ,u_p^i)\in \mathbb {R}^p~\bigg |~A^i+\sum _{k=1}^p u^i_k A^i_k\succeq 0 \right\} , ~i=1,\dots ,l, \end{aligned}$$
(1)

where \(A^i\) and \(A^i_k,\) \(i=1,\dots ,l,\) are symmetric matrices. Note that the spectrahedral uncertain sets in (1) are closed convex sets covering the most commonly used uncertain sets often encountered in robust optimization problems, such as ellipsoids, polytopes, and boxes [14, 29].

The problem \(\text{(UP) }\) with adjustable decision variables can be captured by

$$\begin{aligned} ~~~~~~~~~~~~~~~~~~~~~~\mathrm{{(\text{ UTP})}}~~~~~~~~~~~~~~\left\{ \begin{array}{ll}\mathop {\text{ inf }}\limits _{x\in \mathbb {R}^n,y(\cdot )\in \mathbb {R}^m} ~~f_0(x)+g_0(y(z))\\ ~~~~~~s.t.~~~~~~f_i(x,u^i)+g_i(y(z))\le 0, ~i=1,\dots ,l, \end{array} \right. \end{aligned}$$

where \(g_i(y(z)):=(y(z))^{\top }B_iy(z)+b_i^{\top }y(z)+t_i\), \(i=0,1,\dots ,l,\) with \(B_i:=diag (\theta _1^i,\dots ,\theta _m^i)\in \mathbb {R}^{m\times m},\) \(b_i:=(b^1_i,\dots ,b^m_i)\in \mathbb {R}^m\) and \(t_i\in \mathbb {R}\). Here, \(x:=(x_1,\dots ,x_n)\in \mathbb {R}^n\) is the first-stage (here-and-now) decision variable and \(y(\cdot )\in \mathbb {R}^m\) is the second-stage (wait-and-see) decision variable, which is an adjustable decision variable that depends on the uncertain parameter \(z:=(z_1,\dots ,z_m)\in \mathbb {R}^m\) in a box uncertain set

$$\begin{aligned} \mathcal {U}_{box}:=\prod _{j=1}^m [\beta _j,\gamma _j], \text{ with } \beta _j, \gamma _j\in \mathbb {R} \text{ and } \beta _j\le \gamma _j, ~j=1,\dots ,m. \end{aligned}$$

In what follows, we assume that \(y(\cdot )\) is an affinely adjustable variable in the sense that it satisfies the affine decision rule [5] given by

$$\begin{aligned} y(z):=\rho +W z, \end{aligned}$$
(2)

where \(\rho :=(\rho _1,\dots ,\rho _m)\in \mathbb {R}^m\) and \(W:=diag (\omega _1,\dots ,\omega _m)\in \mathbb {R}^{m\times m}\) are non-adjustable variables.

For (UTP) with the affine decision rule (2), it is usually associated with the robust (worst-case) counterpart given below:

$$\begin{aligned} \mathrm{{(\text{ RTP})}}~~~~~~~~~~~~~~~\left\{ \begin{array}{ll}\mathop {\text{ inf }}\limits _{\begin{array}{c} x\in \mathbb {R}^{n}, \rho \in \mathbb {R}^m,\\ W\in \mathbb {R}^{m\times m} \end{array}} f_0(x)+\mathop {\text{ max }}\limits _{z\in \mathcal {U}_{box}}g_0(y(z)) \\ ~~~~~s.t.~~~~f_i(x,u^i)+g_i(y(z))\le 0, ~\forall u^i\in \mathcal {U}^i,~ i=1,\dots ,l,\\ ~~~~~~~~~~~~~y(z)=\rho +W z, ~\forall z\in \mathcal {U}_{box}. \end{array} \right. \end{aligned}$$

To present the exact SDP reformulation of (RTP), we recall the following results, which play a key role in the sequel.

Lemma 2.1

[12] Let \(\alpha \in \mathbb {R},\) \(v:=(v_1,\dots ,v_m)\in \mathbb {R}^m\) and \(W:=diag (\omega _1,\dots ,\omega _m)\), where \(\omega _j\in \mathbb {R},\) \(j=1,\dots ,m.\) Then, the following statements are equivalent:

(i) The following implication holds

$$\begin{aligned} z:=(z_1,\dots ,z_m)\in \prod _{j=1}^m[\beta _j,\gamma _j]\Rightarrow \alpha +\sum _{j=1}^m v_jz_j+\sum _{j=1}^m \omega _jz_j^2\ge 0, \end{aligned}$$

where \(\beta _j,\gamma _j\in \mathbb {R}\) with \(\beta _j\le \gamma _j,\) \(j=1,\dots ,m.\)

(ii) There exist \(\alpha _j\in \mathbb {R},\) \(j=1,\dots ,m,\) such that \(\sum _{j=1}^m \alpha _j\le \alpha \) and

$$\begin{aligned} \alpha _jh_j^1+v_jh_j^2+\omega _jh_j^3\in \Sigma _4^2[z_j],~j=1,\dots ,m, \end{aligned}$$

where \(\Sigma _4^2[z_j]\) denotes the set consisting of all the sum of squares polynomials with variable \(z_j\) and degree at most 4,  \(h_j^1(z_j):=(1+z_j^2)^2,\) \(h_j^2(z_j):=(\beta _j+\gamma _jz_j^2)(1+z_j^2)\) and \(h_j^3(z_j):=(\beta _j+\gamma _jz_j^2)^2\) for \(z_j\in \mathbb {R}.\)

(iii) There exist \(\alpha _j\in \mathbb {R}\) and \( X^j:= \left( \begin{array}{lll} X_{11}^j ~X_{12}^j ~X_{13}^j\\ X_{12}^j ~X_{22}^j ~X_{23}^j\\ X_{13}^j ~X_{23}^j ~X_{33}^j \end{array}\right) \succeq 0,\) \(j=1,\dots ,m, \) such that \(\sum _{j=1}^m\alpha _j\le \alpha \) and

$$\begin{aligned} \left\{ \begin{aligned}&X_{11}^j=\alpha _j+v_j\beta _j+\omega _j\beta _j^2,\\&X_{12}^j=X_{23}^j=0,\\&2X_{13}^j+X_{22}^j=2\alpha _j+v_j(\beta _j+\gamma _j)+2\omega _j\beta _j\gamma _j,\\&X_{33}^j=\alpha _j+v_j\gamma _j+\omega _j\gamma _j^2. \end{aligned}\right. \end{aligned}$$

3 Exact SDP Reformulations for (RTP)

In this section, we establish an exact SDP reformulation for (RTP) in terms of a special semidefinite representation of the non-negativity of separable non-convex quadratic functions on box uncertain sets. For convenience, let \(u:=(u^1,\dots ,u^l)\in \mathbb {R}^{pl}\) with \(u^i:=(u_1^i,\dots ,u_p^i)\in \mathbb {R}^{p},\) \(\delta :=(\delta _1,\dots ,\delta _l)\in \mathbb {R}^{lm}\) with \(\delta _i:=(\delta ^1_i,\dots ,\delta ^m_i)\in \mathbb {R}^{m},\) \(\sigma :=(\sigma _1,\dots ,\sigma _m)\in \mathbb {R}^m,\) \(X:=(X^{11},\dots ,X^{1m},\dots ,X^{l1},\dots ,X^{lm})\in \mathbb {R}^{3\times 3lm}\) with \(X^{ij}\in \mathbb {S}^3,\) \(Y:=(Y^1,\dots ,Y^m)\in \mathbb {R}^{3\times 3m}\) with \(Y^j\in \mathbb {S}^3,\) \(i=1,\dots ,l,\) \(j=1,\dots ,m.\)

Now, we propose a SDP reformulation for the problem (RTP) as follows:

$$\begin{aligned} \small {{\mathrm {(\text {SDP})}~~~~~~\left\{ \begin{array}{ll} \begin{aligned} &{}\inf \limits _{\begin{array}{c} x\in \mathbb {R}^n, \tau \in \mathbb {R}, \delta \in \mathbb {R}^{lm}, u\in \mathbb {R}^{pl}, \rho \in \mathbb {R}^m, \\ X\in \mathbb {R}^{3\times 3lm}, W\in \mathbb {R}^{m\times m}, \sigma \in \mathbb {R}^m, Y\in \mathbb {R}^{3\times 3m} \end{array}} x^{\top }Q_0x+q_0^{\top }x+\xi _0+\tau \nonumber \\ &{}~~~~~~~~s.t.~~~~\sum _{j=1}^m\delta _i^j\le -\left( \sum _{s=1}^n a_{s}^ix_s^2+\sum _{s=1}^n \left( q^{s}_i+\sum _{k=1}^p u_k^i q^{is}_k\right) x_s+\xi _i+\sum _{k=1}^p u_k^i \xi _k^i\right. \nonumber \\ &{}~~~~~~~~~~~~~~~\left. +\sum _{j=1}^m \theta _j^i \rho _j^2+\sum _{j=1}^m b_i^j \rho _j+t_i\right) ,~i=1,\dots ,l, \nonumber \\ &{}~~~~~~~~~~~~~~~A^i+\sum _{k=1}^p u^i_k A^i_k\succeq 0,~i=1,\dots ,l,\nonumber \\ &{}~~~~~~~~~~~~~~~X^{ij}:= \left( \begin{array}{lll} X_{11}^{ij} ~X_{12}^{ij} ~X_{13}^{ij}\\ X_{12}^{ij} ~X_{22}^{ij} ~X_{23}^{ij}\\ X_{13}^{ij} ~X_{23}^{ij} ~X_{33}^{ij} \end{array}\right) \succeq 0, ~i=1,\dots ,l, ~j=1,\dots ,m,\nonumber \\ &{}~~~~~~~~~~~~~~ \begin{aligned} &{}X_{11}^{ij}=\delta _i^j-\left( 2\rho _j \theta _j^i \omega _j+b_i^j \omega _j\right) \beta _j-\theta _j^i \omega _j^2 \beta _j^2,\\ &{}X_{12}^{ij}=X_{23}^{ij}=0,\\ &{}2X_{13}^{ij}+X_{22}^{ij}=2\delta _i^j-\left( 2\rho _j \theta _j^i \omega _j+b_i^j \omega _j\right) \left( \beta _j+\gamma _j\right) -2 \theta _j^i \omega _j^2 \beta _j \gamma _j,\\ &{}X_{33}^{ij}=\delta _i^j-\left( 2\rho _j \theta _j^i \omega _j+b_i^j \omega _j\right) \gamma _j-\theta _j^i \omega _j^2 \gamma _j^2, \end{aligned} \nonumber \\ &{}~~~~~~~~~~~~~~\sum _{j=1}^m \sigma _j\le \tau -\sum _{j=1}^m \theta _j^0 \rho _j^2-\sum _{j=1}^m b_0^j \rho _j-t_0,\nonumber \\ &{}~~~~~~~~~~~~~~~Y^{j}:= \left( \begin{array}{lll} Y_{11}^{j} ~Y_{12}^{j} ~Y_{13}^{j}\\ Y_{12}^{j} ~Y_{22}^{j} ~Y_{23}^{j}\\ Y_{13}^{j} ~Y_{23}^{j} ~Y_{33}^{j} \end{array}\right) \succeq 0, ~j=1,\dots ,m,\nonumber \\ &{}~~~~~~~~~~~~~~ \begin{aligned} &{}Y_{11}^{j}=\sigma _j-\left( 2\rho _j \theta _j^0 \omega _j+b_0^j \omega _j\right) \beta _j-\theta _j^0 \omega _j^2 \beta _j^2,\\ &{}Y_{12}^{j}=Y_{23}^{j}=0,\\ &{}2Y_{13}^{j}+Y_{22}^j=2\sigma _j-\left( 2\rho _j \theta _j^0 \omega _j+b_0^j \omega _j\right) \left( \beta _j+\gamma _j\right) -2 \theta _j^0 \omega _j^2 \beta _j\gamma _j,\\ &{}Y_{33}^{j}=\sigma _j-\left( 2\rho _j \theta _j^0 \omega _j+b_0^j \omega _j\right) \gamma _j-\theta _j^0 \omega _j^2 \gamma _j^2. \end{aligned} \end{aligned} \end{array} \right. }} \end{aligned}$$

The following theorem describes an exact SDP reformulation for (RTP) in the sense that (RTP) and (SDP) share the same optimal values and their optimal solutions are one-to-one correspondence.

Theorem 3.1

Consider the problem \(\mathrm {(RTP)}\) with \(\mathcal {U}^i,\) \(i=1,\dots ,l,\) given by (1) and its reformulation problem \(\mathrm {(SDP)}\). Then,

$$\begin{aligned} \inf (\textrm{RTP})=\inf (\textrm{SDP}). \end{aligned}$$

Moreover, \((x,\rho ,W)\) is an optimal solution of the problem \(\mathrm {(RTP)}\) if and only if there exist \(\tau \in \mathbb {R},\) \(\delta \in \mathbb {R}^{lm},\) \(u \in \mathbb {R}^{pl},\) \(X \in \mathbb {R}^{3\times 3lm},\) \(\sigma \in \mathbb {R}^m\) and \(Y \in \mathbb {R}^{3\times 3\,m}\) such that \((x,\rho ,W,\tau ,\delta ,u,X,\sigma ,Y)\) is an optimal solution of the problem \(\mathrm {(SDP)}\).

Proof

Obviously, \(\mathrm {(RTP)}\) is equivalent to

$$\begin{aligned} (\text{ RTP}_0)~~~~~~\left\{ \begin{array}{ll} \mathop {\text{ inf }}\limits _{\begin{array}{c} x\in \mathbb {R}^{n}, \tau \in \mathbb {R}, u\in \mathbb {R}^{pl}, \\ \rho \in \mathbb {R}^m, W\in \mathbb {R}^{m\times m} \end{array}} x^{\top }Q_0x+q_0^{\top }x+\xi _0+\tau \nonumber \\ ~~~~~~~s.t.~~~x^{\top }Q_ix+q_i^{\top }x+\xi _i+\sum _{k=1}^p u^i_k\left( (q^i_k)^{\top }x+\xi ^i_k\right) +(y(z))^{\top }B_iy(z)\nonumber \\ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+b_i^{\top }y(z)+t_i\le 0, ~i=1,\dots ,l,\nonumber \\ ~~~~~~~~~~~~~~A^i+\sum _{k=1}^p u^i_k A^i_k\succeq 0,~i=1,\dots ,l,\nonumber \\ ~~~~~~~~~~~~~~(y(z))^{\top }B_0y(z)+b_0^{\top }y(z)+t_0\le \tau ,\nonumber \\ ~~~~~~~~~~~~~~~y(z)=\rho +W z, ~\forall z\in \mathcal {U}_{box}. \end{array} \right. \end{aligned}$$

Note that \(\tau \) is an auxiliary variable and adding it does not cause the optimal value to change. Therefore, it is sufficient to show that the feasible sets between \(\mathrm {(RTP_0)}\) and \(\mathrm {(SDP)}\) are equivalent.

Now, let \((x,\rho ,W,\tau ,u )\) be a feasible solution of \(\mathrm {(RTP_0)}\). This means that

$$\begin{aligned}{} & {} x^{\top }Q_i x+q_i^{\top }x+\xi _i+\sum _{k=1}^p u^i_k\left( (q^i_k)^{\top } x +\xi ^i_k\right) \nonumber \\{} & {} ~~~~~+(y(z))^{\top }B_iy(z)+b_i^{\top }y(z)+t_i\le 0,~ i=1,\dots ,l, \end{aligned}$$
(3)
$$\begin{aligned}{} & {} A^i+\sum _{k=1}^p u^i_k A^i_k\succeq 0,~i=1,\dots ,l,\nonumber \\{} & {} (y(z))^{\top }B_0y(z)+b_0^{\top }y(z)+t_0\le \tau , \end{aligned}$$
(4)

and

$$\begin{aligned} y(z)=\rho +W z, ~\forall z\in \mathcal {U}_{box}. \end{aligned}$$
(5)

By (3) and (5), we obtain

$$\begin{aligned}{} & {} -\left( \sum _{s=1}^n a_{s}^i(x_s)^2+\sum _{s=1}^n \left( q^{s}_i+\sum _{k=1}^p u_k^i q^{is}_k\right) x_s+\xi _i+\sum _{k=1}^p u_k^i \xi _k^i+\sum _{j=1}^m \theta _j^i (\rho _j)^2+\sum _{j=1}^m b_i^j \rho _j+t_i\right) \nonumber \\{} & {} -\left( \sum _{j=1}^m 2\rho _j \theta _j^i \omega _j+\sum _{j=1}^m b_i^j \omega _j\right) z_j-\sum _{j=1}^m \theta _j^i (\omega _j)^2 z_j^2\ge 0, ~\forall z\in \mathcal {U}_{box}, i=1,\dots ,l. \end{aligned}$$
(6)

From Lemma 2.1 and (6), there exist \(\delta _i^j\in \mathbb {R}\) and \( X^{ij}:= \left( \begin{array}{lll} X_{11}^{ij} ~X_{12}^{ij} ~X_{13}^{ij}\\ X_{12}^{ij} ~X_{22}^{ij} ~X_{23}^{ij}\\ X_{13}^{ij} ~X_{23}^{ij} ~X_{33}^{ij} \end{array}\right) \succeq 0, ~i=1,\dots ,l, ~j=1,\dots ,m, \) such that

$$\begin{aligned}{} & {} \sum _{j=1}^m \delta _i^j \le -\left( \sum _{s=1}^n a_{s}^i(x_s)^2+\sum _{s=1}^n \left( q^{s}_i+\sum _{k=1}^p u_k^i q^{is}_k\right) x_s+\xi _i\right. \nonumber \\{} & {} ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~\left. +\sum _{k=1}^p u_k^i \xi _k^i+\sum _{j=1}^m \theta _j^i (\rho _j)^2+\sum _{j=1}^m b_i^j \rho _j+t_i\right) , \end{aligned}$$
(7)

and

$$\begin{aligned} \left\{ \begin{aligned}&X_{11}^{ij}=\delta _i^j-\left( 2\rho _j \theta _j^i \omega _j+b_i^j \omega _j\right) \beta _j-\theta _j^i (\omega _j)^2 \beta _j^2,\\&X_{12}^{ij}=X_{23}^{ij}=0,\\&2X_{13}^{ij}+X_{22}^{ij}=2\delta _i^j-\left( 2\rho _j \theta _j^i \omega _j+b_i^j \omega _j\right) \left( \beta _j+\gamma _j\right) -2 \theta _j^i (\omega _j)^2 \beta _j\gamma _j,\\&X_{33}^{ij}=\delta _i^j-\left( 2\rho _j \theta _j^i \omega _j+b_i^j \omega _j\right) \gamma _j-\theta _j^i (\omega _j)^2 \gamma _j^2. \end{aligned}\right. \end{aligned}$$
(8)

Moreover, (4) and (5) amount to

$$\begin{aligned}{} & {} \tau -\sum _{j=1}^m \theta _j^0 (\rho _j)^2-\sum _{j=1}^m b_0^j \rho _j-t_0-\left( \sum _{j=1}^m 2\rho _j \theta _j^0 \omega _j+\sum _{j=1}^m b_0^j \omega _j\right) z_j\nonumber \\{} & {} ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~-\sum _{j=1}^m \theta _j^0 (\omega _j)^2 z_j^2\ge 0, ~\forall z\in \mathcal {U}_{box}. \end{aligned}$$
(9)

Similarly, from Lemma 2.1 and (9), there exist \(\sigma _j\in \mathbb {R}\) and \( Y^{j}:= \left( \begin{array}{lll} Y_{11}^{j} ~Y_{12}^{j} ~Y_{13}^{j}\\ Y_{12}^{j} ~Y_{22}^{j} ~Y_{23}^{j}\\ Y_{13}^{j} ~Y_{23}^{j} ~Y_{33}^{j} \end{array}\right) \succeq 0, ~j=1,\dots ,m, \) such that

$$\begin{aligned} \sum _{j=1}^m\sigma _j\le \tau -\sum _{j=1}^m \theta _j^0 (\rho _j)^2-\sum _{j=1}^m b_0^j \rho _j-t_0, \end{aligned}$$
(10)

and

$$\begin{aligned} \left\{ \begin{aligned}&Y_{11}^{j}=\sigma _j-\left( 2\rho _j \theta _j^0 \omega _j+ b_0^j \omega _j\right) \beta _j-\theta _j^0 (\omega _j)^2 \beta _j^2,\\&Y_{12}^{j}=Y_{23}^{j}=0,\\&2Y_{13}^{j}+Y_{22}^j=2\sigma _j-\left( 2\rho _j \theta _j^0 \omega _j+ b_0^j \omega _j\right) \left( \beta _j+\gamma _j\right) -2 \theta _j^0 (\omega _j)^2 \beta _j\gamma _j,\\&Y_{33}^{j}=\sigma _j-\left( 2\rho _j \theta _j^0 \omega _j+ b_0^j \omega _j\right) \gamma _j-\theta _j^0 (\omega _j)^2 \gamma _j^2. \end{aligned}\right. \end{aligned}$$
(11)

Together with (7), (8), (10) and (11), it follows that the problems \(\mathrm {(RTP)}\) and \(\mathrm {(SDP)}\) are equivalent in the sense that

$$\begin{aligned} \inf (\textrm{RTP})=\inf (\textrm{SDP}). \end{aligned}$$

Moreover, \((x,\rho ,W)\) is a feasible solution of the problem \(\mathrm {(RTP)}\) if and only if there exist \(\tau \in \mathbb {R},\) \(\delta \in \mathbb {R}^{lm},\) \(u \in \mathbb {R}^{pl},\) \(X \in \mathbb {R}^{3\times 3lm},\) \(\sigma \in \mathbb {R}^m\) and \(Y \in \mathbb {R}^{3\times 3\,m}\) such that \((x,\rho ,W,\tau ,\delta ,u,X,\sigma ,Y)\) is an optimal solution of the problem \(\mathrm {(SDP)}\). The proof is complete.

Remark 3.1

Note that similar results for exact SDP reformulations of adjustable robust linear optimization problems have been investigated in [12, Theorem 3.1]. However, Theorem 3.1 extends these results from linear optimization models to quadratic optimization models.

The following example illustrates how to obtain robust optimal solutions and the corresponding optimal value of (RTP) on spectrahedral uncertain sets by Theorem 3.1.

Example 3.1

For problem (UTP). Let \(n=p=m:=2\) and \(l:=4.\) The uncertain sets \(\mathcal {U}^i,\) \(i=1,\dots ,4\), are defined by

$$\begin{aligned} \mathcal {U}^i:=\left\{ u^i:=(u^i_1,u^i_2)\in \mathbb {R}^2~|~\frac{(u^i_1)^2}{2}+\frac{(u^i_2)^2}{3}\le 1\right\} , i=1,\dots ,4. \end{aligned}$$

Obviously, by (1), we have

$$\begin{aligned} A^i=\left( \begin{array}{lll} 2 &{} 0 &{} 0\\ 0 &{} 3 &{} 0\\ 0 &{} 0 &{} 1 \end{array}\right) , A_{1}^i=\left( \begin{array}{lll} 0 &{} 0 &{} 1\\ 0 &{} 0 &{} 0\\ 1 &{} 0 &{} 0 \end{array}\right) \text{ and } A_{2}^i=\left( \begin{array}{lll} 0 &{} 0 &{} 0\\ 0 &{} 0 &{} 1\\ 0 &{} 1 &{} 0 \end{array}\right) , i=1,\dots ,4. \end{aligned}$$

Let \(f_0(x):=x_1+x_2\), \(g_0(y(z)):=0,\) \(f_1(x,u^1):=x_1^2-3x_1-x_2+u^1_1\), \(g_1(y(z)):=\left( 0,-1\right) ^{\top }y(z)-3,\) \(f_2(x,u^2):=-2x_2\), \(g_2(y(z)):=2,\) \(f_3(x,u^3):= -3x_1 \), \(g_3(y(z)):= 3,\) \(f_4(x,u^4):=x_2^2-x_1-u^4_2\), and \(g_4(y(z)):=\left( 1,0\right) ^{\top }y(z).\) Let the affine decision rule \(y(\cdot )\) be given by \( y(z):=\rho +W z, \) where \(\rho :=(\rho _1,\rho _2)\in \mathbb {R}^2,\) \(W:=diag (\omega _1,\omega _2)\in \mathbb {R}^{2\times 2}\) and \(z:=(z_1,z_2)\in \mathcal {U}_{box}:=[-1,1]\times [0,2]\). In this setting, (RTP) becomes

$$\begin{aligned} { \left\{ \begin{array}{ll} \mathop {\text{ inf }}\limits _{\begin{array}{c} x\in \mathbb {R}^{2}, \rho \in \mathbb {R}^2, \\ W\in \mathbb {R}^{2\times 2} \end{array}} ~x_1+x_2\\ ~~~~s.t.~~~~~x_1^2-3x_1-x_2+u^1_1-\rho _2-\omega _2 z_2-3\le 0,\\ ~~~~~~~~~~~~-2x_2+2 \le 0, \\ ~~~~~~~~~~~~-3x_1+3 \le 0,\\ ~~~~~~~~~~~~~x_2^2-x_1-u^4_2+\rho _1+\omega _1 z_1\le 0,\\ ~~~~~~~~~~~~~\forall (z_1,z_2)\in \mathcal {U}_{box}, \forall (u^i_1,u^i_2)\in \mathcal {U}^i,i=1,\dots ,4. \end{array}\right. } \end{aligned}$$

It is easy to show that the problem (RTP) admits an optimal solution \(( {x}, {\rho }, {W})\) with \( {x}=(1,1),\) \( {\rho }=(-151,151)\) and \( {W}=\text{ O}_{2\times 2}.\) Moreover, \(\min \mathrm {(RTP)}=2.\)

Now, we apply Theorem 3.1 to show that \(( {x}, {\rho }, {W})\) is an optimal solution of (RTP). Clearly, the SDP reformulation of (RTP) becomes

$$\begin{aligned} { \left\{ \begin{array}{ll} \begin{aligned} &{}\inf \limits _{\begin{array}{c} x\in \mathbb {R}^2, \delta \in \mathbb {R}^{8}, u\in \mathbb {R}^{8}, \\ \rho \in \mathbb {R}^2, X\in \mathbb {R}^{3\times 24}, W\in \mathbb {R}^{2\times 2} \end{array}} x_1+x_2\nonumber \\ &{}~~~~~~~~s.t.~~~~\sum _{j=1}^2\delta _i^j\le -\left( \sum _{s=1}^2 a_{s}^ix_s^2+\sum _{s=1}^2 \left( q^{s}_i+\sum _{k=1}^2 u_k^i q^{is}_k\right) x_s+\xi _i+\sum _{k=1}^2 u_k^i \xi _k^i\right. \nonumber \\ &{}~~~~~~~~~~~~~~~\left. +\sum _{j=1}^2 \theta _j^i \rho _j^2+\sum _{j=1}^2 b_i^j \rho _j+t_i\right) ,~i=1,\dots ,4, \nonumber \\ &{}~~~~~~~~~~~~~~~A^i+\sum _{k=1}^2 u^i_k A^i_k\succeq 0,~i=1,\dots ,4,\nonumber \\ &{}~~~~~~~~~~~~~~~X^{ij}:= \left( \begin{array}{lll} X_{11}^{ij} ~X_{12}^{ij} ~X_{13}^{ij}\\ X_{12}^{ij} ~X_{22}^{ij} ~X_{23}^{ij}\\ X_{13}^{ij} ~X_{23}^{ij} ~X_{33}^{ij} \end{array}\right) \succeq 0, ~i=1,\dots ,4, ~j=1,2,\nonumber \\ &{}~~~~~~~~~~~~~~ \begin{aligned} &{}X_{11}^{ij}=\delta _i^j-\left( 2\rho _j \theta _j^i \omega _j+b_i^j \omega _j\right) \beta _j-\theta _j^i \omega _j^2 \beta _j^2,\\ &{}X_{12}^{ij}=X_{23}^{ij}=0,\\ &{}2X_{13}^{ij}+X_{22}^{ij}=2\delta _i^j-\left( 2\rho _j \theta _j^i \omega _j+b_i^j \omega _j\right) \left( \beta _j+\gamma _j\right) -2 \theta _j^i \omega _j^2 \beta _j\gamma _j,\\ &{}X_{33}^{ij}=\delta _i^j-\left( 2\rho _j \theta _j^i \omega _j+b_i^j \omega _j\right) \gamma _j-\theta _j^i \omega _j^2 \gamma _j^2. \end{aligned} \end{aligned} \end{array}\right. } \end{aligned}$$

Using the Matlab toolbox CVX [18], we solve (SDP). The solver returns \(\min \mathrm {(RTP)}=2\) and an optimal solution of (SDP) as \((x,\rho ,W,\delta ,u,X)\), where \(x=(1.0000,1.0000),\) \(\rho =(-151.0951, 151.4487),\) \(W=\left( \begin{array}{lll} 0.1148 &{} ~0.0000 \\ 0.0000 &{} -0.1148 \end{array}\right) ,\) \(\delta =(50.1210,50.0255,4.9909\times 10^{-10},1.4983\times 10^{-9},2.2255\times 10^{-9},2.2255\times 10^{-9},49.9106,50.1210)\), \(u=(u^1,u^2,u^3,u^4)\) with \(u^i=(-1.9376\times 10^{-20},-2.7868\times 10^{-20}),\) \(i=1,\dots ,4,\) and \(X:=(X^{11},X^{12},X^{21},X^{22},X^{31},X^{32},X^{41},X^{42})\) with \( X^{11}=\left( \begin{array}{lll} 50.1210 &{}~0.000 &{}~10.0608\\ 0.0000 &{}~80.1204 &{}~0.0000\\ 10.0608 &{}~0.0000 &{}~50.1210 \end{array}\right) ,\) \( X^{12}=\left( \begin{array}{lll} 50.0255 &{}~0.000 &{}~9.9271 \\ 0.0000 &{}~79.9670 &{}~0.0000\\ 9.9271 &{}~0.0000 &{}~49.7958 \end{array}\right) ,~~~~~\) Thus, by Theorem 3.1, \(( {x}, {\rho }, {W})\) is an optimal solution of (RTP).

Now, let us consider special cases of (RTP). In the special case when \(B_0=\text{ O}_{m\times m},\) \(b_0=0_m\) and \(t_0=0\), (RTP) collapses to

$$\begin{aligned} { (\text{ RTP}_1)~~~~~~~\left\{ \begin{array}{ll} \mathop {\text{ inf }}\limits _{\begin{array}{c} x\in \mathbb {R}^{n}, \rho \in \mathbb {R}^m, \\ W\in \mathbb {R}^{m\times m} \end{array}} x^{\top }Q_0x+q_0^{\top }x+\xi _0\\ ~~~~~s.t.~~~~x^{\top }Q_ix+q_i^{\top }x+\xi _i+\sum _{k=1}^p u^i_k\left( (q^i_k)^{\top }x+\xi ^i_k\right) +(y(z))^{\top }B_iy(z)\\ ~~~~~~~~~~~~~+b_i^{\top }y(z)+t_i\le 0, ~\forall (u^i_1,\dots ,u^i_p)\in \mathcal {U}^i,~i=1,\dots ,l,\\ ~~~~~~~~~~~~~y(z)=\rho +W z, ~\forall z\in \mathcal {U}_{box}, \end{array} \right. } \end{aligned}$$

and its SDP reformulation becomes

$$\begin{aligned} { (\text{ SDP}_1)~~~~~~~~~\left\{ \begin{array}{ll} \begin{aligned} &{}\mathop {\text{ inf }}\limits _{\begin{array}{c} x\in \mathbb {R}^n, \delta \in \mathbb {R}^{lm}, u\in \mathbb {R}^{pl}, \rho \in \mathbb {R}^m, \\ X\in \mathbb {R}^{3\times 3lm}, W\in \mathbb {R}^{m\times m} \end{array}} x^{\top }Q_0x+q_0^{\top }x+\xi _0\nonumber \\ &{}~~~~~~~~s.t.~~~~\sum _{j=1}^m\delta _i^j\le -\left( \sum _{s=1}^n a_{s}^ix_s^2+\sum _{s=1}^n \left( q^{s}_i+\sum _{k=1}^p u_k^i q^{is}_k\right) x_s+\xi _i+\sum _{k=1}^p u_k^i \xi _k^i\right. \nonumber \\ &{}~~~~~~~~~~~~~~~\left. +\sum _{j=1}^m \theta _j^i \rho _j^2+\sum _{j=1}^m b_i^j \rho _j+t_i\right) ,~i=1,\dots ,l, \nonumber \\ &{}~~~~~~~~~~~~~~~A^i+\sum _{k=1}^p u^i_k A^i_k\succeq 0,~i=1,\dots ,l,\nonumber \\ &{}~~~~~~~~~~~~~~~X^{ij}:= \left( \begin{array}{lll} X_{11}^{ij} ~X_{12}^{ij} ~X_{13}^{ij}\\ X_{12}^{ij} ~X_{22}^{ij} ~X_{23}^{ij}\\ X_{13}^{ij} ~X_{23}^{ij} ~X_{33}^{ij} \end{array}\right) \succeq 0, ~i=1,\dots ,l, ~j=1,\dots ,m,\nonumber \\ &{}~~~~~~~~~~~~~~ \begin{aligned} &{}X_{11}^{ij}=\delta _i^j-\left( 2\rho _j \theta _j^i \omega _j+b_i^j \omega _j\right) \beta _j-\theta _j^i \omega _j^2 \beta _j^2,\\ &{}X_{12}^{ij}=X_{23}^{ij}=0,\\ &{}2X_{13}^{ij}+X_{22}^{ij}=2\delta _i^j-\left( 2\rho _j \theta _j^i \omega _j+b_i^j \omega _j\right) \left( \beta _j+\gamma _j\right) -2 \theta _j^i \omega _j^2 \beta _j\gamma _j,\\ &{}X_{33}^{ij}=\delta _i^j-\left( 2\rho _j \theta _j^i \omega _j+b_i^j \omega _j\right) \gamma _j-\theta _j^i \omega _j^2 \gamma _j^2. \end{aligned} \end{aligned} \end{array} \right. } \end{aligned}$$

By virtue of Theorem 3.1, we can easily obtain the following result.

Corollary 3.1

Consider the problem \(\mathrm {(RTP_1)}\) with \(\mathcal {U}^i,\) \(i=1,\dots ,l,\) given by (1) and its reformulation problem \(\mathrm {(SDP_1)}\). Then,

$$\begin{aligned} \inf (\textrm{RTP}_1)=\inf (\textrm{SDP}_1). \end{aligned}$$

Moreover, \((x,\rho ,W)\) is an optimal solution of \(\mathrm {(RTP_1)}\) if and only if there exist \(\delta \in \mathbb {R}^{lm},\) \(u \in \mathbb {R}^{pl}\) and \(X \in \mathbb {R}^{3\times 3lm}\) such that \(\left( x,\rho ,W,\delta ,u,X \right) \) is an optimal solution of the problem \(\mathrm {(SDP_1)}\).

In the special case when (RTP) with \(t_0=0\) and \(Q_i=B_i=\text{ O}_{m\times m},\) \(i = 0,1,\dots ,l,\) (RTP) collapses to the following adjustable robust linear optimization

$$\begin{aligned} (\text{ RTP}_2)~~~~~~~~~~~~~~\left\{ \begin{array}{ll} \mathop {\text{ inf }}\limits _{\begin{array}{c} x\in \mathbb {R}^{n}, \rho \in \mathbb {R}^m,\\ W\in \mathbb {R}^{m\times m} \end{array}} ~q_0^{\top }x+\xi _0+\mathop {\text{ max }}\limits _{z\in \mathcal {U}_{box}} b_0^{\top }y(z)\\ ~~~~~s.t.~~~~~q_i^{\top }x+\xi _i+\sum _{k=1}^p u^i_k\left( (q^i_k)^{\top }x+\xi ^i_k\right) +b_i^{\top }y(z)\\ ~~~~~~~~~~~~~+t_i\le 0, ~\forall (u_1^i,\dots ,u_p^i)\in \mathcal {U}^i,~ i=1,\dots ,l,\\ ~~~~~~~~~~~~~~y(z)=\rho +W z, ~\forall z\in \mathcal {U}_{box}, \end{array} \right. \end{aligned}$$

and its SDP reformulation becomes

$$\begin{aligned} { (\text{ SDP}_2)~~~~~~~~\left\{ \begin{array}{ll} \begin{aligned} &{}\inf \limits _{\begin{array}{c} x\in \mathbb {R}^n, \tau \in \mathbb {R}, \delta \in \mathbb {R}^{lm}, u\in \mathbb {R}^{pl}, \rho \in \mathbb {R}^m, \\ X\in \mathbb {R}^{3\times 3lm}, W\in \mathbb {R}^{m\times m}, \sigma \in \mathbb {R}^m, Y\in \mathbb {R}^{3\times 3m} \end{array}} q_0^{\top }x+\xi _0+\tau \nonumber \\ &{}~~~~~~~~s.t.~~~~\sum _{j=1}^m\delta _i^j\le -\left( \sum _{s=1}^n \left( q^{s}_i+\sum _{k=1}^p u_k^i q^{is}_k\right) x_s+\xi _i+\sum _{k=1}^p u_k^i \xi _k^i\right. \nonumber \\ &{}~~~~~~~~~~~~~~~\left. +\sum _{j=1}^m b_i^j \rho _j+t_i\right) ,~i=1,\dots ,l, \nonumber \\ &{}~~~~~~~~~~~~~~~A^i+\sum _{k=1}^p u^i_k A^i_k\succeq 0,~i=1,\dots ,l,\nonumber \\ &{}~~~~~~~~~~~~~~~X^{ij}:= \left( \begin{array}{lll} X_{11}^{ij} ~X_{12}^{ij} ~X_{13}^{ij}\\ X_{12}^{ij} ~X_{22}^{ij} ~X_{23}^{ij}\\ X_{13}^{ij} ~X_{23}^{ij} ~X_{33}^{ij} \end{array}\right) \succeq 0, ~i=1,\dots ,l, ~j=1,\dots ,m,\nonumber \\ &{}~~~~~~~~~~~~~~ \begin{aligned} &{}X_{11}^{ij}=\delta _i^j-b_i^j \omega _j \beta _j,\\ &{}X_{12}^{ij}=X_{23}^{ij}=0,\\ &{}2X_{13}^{ij}+X_{22}^{ij}=2\delta _i^j-b_i^j \omega _j \left( \beta _j+\gamma _j\right) ,\\ &{}X_{33}^{ij}=\delta _i^j-b_i^j \omega _j \gamma _j, \end{aligned} \nonumber \\ &{}~~~~~~~~~~~~~~\sum _{j=1}^m \sigma _j\le \tau -\sum _{j=1}^m b_0^j \rho _j,\nonumber \\ &{}~~~~~~~~~~~~~~~Y^{j}:= \left( \begin{array}{lll} Y_{11}^{j} ~Y_{12}^{j} ~Y_{13}^{j}\\ Y_{12}^{j} ~Y_{22}^{j} ~Y_{23}^{j}\\ Y_{13}^{j} ~Y_{23}^{j} ~Y_{33}^{j} \end{array}\right) \succeq 0, ~j=1,\dots ,m,\nonumber \\ &{}~~~~~~~~~~~~~~ \begin{aligned} &{}Y_{11}^{j}=\sigma _j-b_0^j \omega _j \beta _j,\\ &{}Y_{12}^{j}=Y_{23}^{j}=0,\\ &{}2Y_{13}^{j}+Y_{22}^j=2\sigma _j-b_0^j \omega _j \left( \beta _j+\gamma _j\right) ,\\ &{}Y_{33}^{j}=\sigma _j-b_0^j \omega _j\gamma _j. \end{aligned} \end{aligned} \end{array} \right. } \end{aligned}$$

Corollary 3.2

Consider the problem \(\mathrm {(RTP_2)}\) with \(\mathcal {U}^i,\) \(i=1,\dots ,l,\) given by (1) and its reformulation problem \(\mathrm {(SDP_2)}\). Then,

$$\begin{aligned} \inf (\textrm{RTP}_2)=\inf (\textrm{SDP}_2). \end{aligned}$$

Moreover, \((x,\rho ,W)\) is an optimal solution of the problem \(\mathrm {(RTP_2)}\) if and only if there exist \(\tau \in \mathbb {R},\) \(\delta \in \mathbb {R}^{lm},\) \(u \in \mathbb {R}^{pl},\) \(X \in \mathbb {R}^{3\times 3lm},\) \(\sigma \in \mathbb {R}^m\) and \(Y \in \mathbb {R}^{3\times 3\,m}\) such that \((x,\rho ,W,\tau ,\delta ,u,X,\sigma ,Y)\) is an optimal solution of the problem \(\mathrm {(SDP_2)}\).

Remark 3.2

Corollary 3.2 improves upon the results obtained in [32, Theorem 2.3], where no adjustable variables are involved in the objective function of the adjustable robust linear optimization problem.

At the end of this section, we examine the category of uncertain sets such that (RTP) enjoys the SDP reformulation. We first consider the case when \(\mathcal {U}^i\), \(i=1,\dots ,l,\) are ellipsoidal uncertain sets, that is, \(\mathcal {{U}}^i\) is given by

$$\begin{aligned} \mathcal {{U}}^i:=\left\{ u^i:=(u_1^i,\dots ,u_p^i)\in \mathbb {R}^p~\bigg |~ (u^i)^{\top } E^i u^i\le 1 \right\} , ~i=1,\dots ,l. \end{aligned}$$
(12)

Here, \(E^i\in \mathbb {S}^p\) and \(E^i\succ 0,\) \(i=1,\dots ,l.\)

In this case, its SDP reformulation becomes

$$\begin{aligned} { \left\{ \begin{array}{ll} \begin{aligned} &{}\inf \limits _{\begin{array}{c} x\in \mathbb {R}^n, \tau \in \mathbb {R}, \delta \in \mathbb {R}^{lm}, u\in \mathbb {R}^{pl}, \rho \in \mathbb {R}^m, \\ X\in \mathbb {R}^{3\times 3lm}, W\in \mathbb {R}^{m\times m}, \sigma \in \mathbb {R}^m, Y\in \mathbb {R}^{3\times 3m} \end{array}} x^{\top }Q_0x+q_0^{\top }x+\xi _0+\tau \nonumber \\ &{}~~~~~~~~s.t.~~~~\sum _{j=1}^m\delta _i^j\le -\left( \sum _{s=1}^n a_{s}^ix_s^2+\sum _{s=1}^n \left( q^{s}_i+\sum _{k=1}^p u_k^i q^{is}_k\right) x_s+\xi _i+\sum _{k=1}^p u_k^i \xi _k^i\right. \nonumber \\ &{}~~~~~~~~~~~~~~~\left. +\sum _{j=1}^m \theta _j^i \rho _j^2+\sum _{j=1}^m b_i^j \rho _j+t_i\right) ,~i=1,\dots ,l, \nonumber \\ &{}~~~~~~~~~~~~~~~ \left( \begin{array}{lll} (E^i)^{-1} &{}u^i\\ (u^i)^{\top } &{}1 \end{array}\right) \succeq 0, ~i=1,\dots ,l,\nonumber \\ &{}~~~~~~~~~~~~~~~X^{ij}:= \left( \begin{array}{lll} X_{11}^{ij} ~X_{12}^{ij} ~X_{13}^{ij}\\ X_{12}^{ij} ~X_{22}^{ij} ~X_{23}^{ij}\\ X_{13}^{ij} ~X_{23}^{ij} ~X_{33}^{ij} \end{array}\right) \succeq 0, ~i=1,\dots ,l, ~j=1,\dots ,m,\nonumber \\ &{}~~~~~~~~~~~~~~ \begin{aligned} &{}X_{11}^{ij}=\delta _i^j-\left( 2\rho _j \theta _j^i \omega _j+b_i^j \omega _j\right) \beta _j-\theta _j^i \omega _j^2 \beta _j^2,\\ &{}X_{12}^{ij}=X_{23}^{ij}=0,\\ &{}2X_{13}^{ij}+X_{22}^{ij}=2\delta _i^j-\left( 2\rho _j \theta _j^i \omega _j+b_i^j \omega _j\right) \left( \beta _j+\gamma _j\right) -2 \theta _j^i \omega _j^2 \beta _j\gamma _j,\\ &{}X_{33}^{ij}=\delta _i^j-\left( 2\rho _j \theta _j^i \omega _j+b_i^j \omega _j\right) \gamma _j-\theta _j^i \omega _j^2 \gamma _j^2, \end{aligned} \nonumber \\ &{}~~~~~~~~~~~~~~\sum _{j=1}^m \sigma _j\le \tau -\sum _{j=1}^m \theta _j^0 \rho _j^2-\sum _{j=1}^m b_0^j \rho _j-t_0,\nonumber \\ &{}~~~~~~~~~~~~~~~Y^{j}:= \left( \begin{array}{lll} Y_{11}^{j} ~Y_{12}^{j} ~Y_{13}^{j}\\ Y_{12}^{j} ~Y_{22}^{j} ~Y_{23}^{j}\\ Y_{13}^{j} ~Y_{23}^{j} ~Y_{33}^{j} \end{array}\right) \succeq 0, ~j=1,\dots ,m,\nonumber \\ &{}~~~~~~~~~~~~~~ \begin{aligned} &{}Y_{11}^{j}=\sigma _j-\left( 2\rho _j \theta _j^0 \omega _j+b_0^j \omega _j\right) \beta _j-\theta _j^0 \omega _j^2 \beta _j^2,\\ &{}Y_{12}^{j}=Y_{23}^{j}=0,\\ &{}2Y_{13}^{j}+Y_{22}^j=2\sigma _j-\left( 2\rho _j \theta _j^0 \omega _j+b_0^j \omega _j\right) \left( \beta _j+\gamma _j\right) -2 \theta _j^0 \omega _j^2 \beta _j\gamma _j,\\ &{}Y_{33}^{j}=\sigma _j-\left( 2\rho _j \theta _j^0 \omega _j+b_0^j \omega _j\right) \gamma _j-\theta _j^0 \omega _j^2 \gamma _j^2. \end{aligned} \end{aligned} \end{array} \right. } \end{aligned}$$

Corollary 3.3

Consider the problem \(\mathrm {(RTP)}\) with \(\mathcal {U}^i,\) \(i=1,\dots ,l,\) given by (12) and its reformulation problem \(\mathrm {(SDP)}\). Then,

$$\begin{aligned} \inf (\textrm{RTP})=\inf (\textrm{SDP}). \end{aligned}$$

Moreover, \((x,\rho ,W)\) is an optimal solution of the problem \(\mathrm {(RTP)}\) if and only if there exist \(\tau \in \mathbb {R},\) \(\delta \in \mathbb {R}^{lm},\) \(u \in \mathbb {R}^{pl},\) \(X \in \mathbb {R}^{3\times 3lm},\) \(\sigma \in \mathbb {R}^m\) and \(Y \in \mathbb {R}^{3\times 3\,m}\) such that \((x,\rho ,W,\tau ,\delta ,u,X,\sigma ,Y)\) is an optimal solution of the problem \(\mathrm {(SDP)}\).

Proof

Following similar arguments to those used in [14, Corollary 2.1], we can show that

$$\begin{aligned} A^i+\sum _{k=1}^p u^i_k A^i_k\succeq 0 \Longleftrightarrow \left( \begin{array}{lll} (E^i)^{-1} &{}u^i\\ (u^i)^{\top } &{}1 \end{array}\right) \succeq 0 \Longleftrightarrow 1-(u^i)^{\top } E^i u^i\ge 0,~i=1,\dots ,l.\nonumber \\ \end{aligned}$$
(13)

Then, the desired result is obtained.

Remark 3.3

Note that related results for SDP reformulations of static robust quadratic optimization problems on ellipsoidal uncertain sets have been investigated in [26, Lemma 2]. However, Corollary 3.3 extends these results from static to the adjustable setting.

We now consider the case where \(\mathcal {U}^i,\) \(i=1,\dots ,l,\) are cross-polytopes defined by

$$\begin{aligned} \mathcal {U}^i:=\left\{ u^i:=(u_1^i,\dots ,u_p^i)\in \mathbb {R}^p~\bigg |~ \sum _{k=1}^p |u_k^i|\le \lambda _i \right\} , ~i=1,\dots ,l, \end{aligned}$$
(14)

where \(\lambda _i>0,\) \(i=1,\dots ,l.\)

In this case, the SDP reformulation of \(\mathrm {(RTP)}\) becomes

$$\begin{aligned} { \left\{ \begin{array}{ll} \begin{aligned} &{}\inf \limits _{\begin{array}{c} x\in \mathbb {R}^n, \tau \in \mathbb {R}, \delta \in \mathbb {R}^{lm}, u\in \mathbb {R}^{pl}, \rho \in \mathbb {R}^m, \\ X\in \mathbb {R}^{3\times 3lm}, W\in \mathbb {R}^{m\times m}, \sigma \in \mathbb {R}^m, Y\in \mathbb {R}^{3\times 3m} \end{array}} x^{\top }Q_0x+q_0^{\top }x+\xi _0+\tau \nonumber \\ &{}~~~~~~~~s.t.~~~~\sum _{j=1}^m\delta _i^j\le -\left( \sum _{s=1}^n a_{s}^ix_s^2+\sum _{s=1}^n \left( q^{s}_i+\sum _{k=1}^p u_k^i q^{is}_k\right) x_s+\xi _i+\sum _{k=1}^p u_k^i \xi _k^i\right. \nonumber \\ &{}~~~~~~~~~~~~~~~\left. +\sum _{j=1}^m \theta _j^i \rho _j^2+\sum _{j=1}^m b_i^j \rho _j+t_i\right) ,~i=1,\dots ,l, \nonumber \\ &{}~~~~~~~~~~~~~~~~\lambda _i + \sum _{k=1}^p u_k^i\ge 0,~\lambda _i - \sum _{k=1}^p u_k^i\ge 0,~i=1,\dots ,l,\nonumber \\ &{}~~~~~~~~~~~~~~~~X^{ij}:= \left( \begin{array}{lll} X_{11}^{ij} ~X_{12}^{ij} ~X_{13}^{ij}\\ X_{12}^{ij} ~X_{22}^{ij} ~X_{23}^{ij}\\ X_{13}^{ij} ~X_{23}^{ij} ~X_{33}^{ij} \end{array}\right) \succeq 0, ~i=1,\dots ,l, ~j=1,\dots ,m,\nonumber \\ &{}~~~~~~~~~~~~~~~ \begin{aligned} &{}X_{11}^{ij}=\delta _i^j-\left( 2\rho _j \theta _j^i \omega _j+b_i^j \omega _j\right) \beta _j-\theta _j^i \omega _j^2 \beta _j^2,\\ &{}X_{12}^{ij}=X_{23}^{ij}=0,\\ &{}2X_{13}^{ij}+X_{22}^{ij}=2\delta _i^j-\left( 2\rho _j \theta _j^i \omega _j+b_i^j \omega _j\right) \left( \beta _j+\gamma _j\right) -2 \theta _j^i \omega _j^2 \beta _j\gamma _j,\\ &{}X_{33}^{ij}=\delta _i^j-\left( 2\rho _j \theta _j^i \omega _j+b_i^j \omega _j\right) \gamma _j-\theta _j^i \omega _j^2 \gamma _j^2, \end{aligned} \nonumber \\ &{}~~~~~~~~~~~~~~~\sum _{j=1}^m \sigma _j\le \tau -\sum _{j=1}^m \theta _j^0 \rho _j^2-\sum _{j=1}^m b_0^j \rho _j-t_0,\nonumber \\ &{}~~~~~~~~~~~~~~~~Y^{j}:= \left( \begin{array}{lll} Y_{11}^{j} ~Y_{12}^{j} ~Y_{13}^{j}\\ Y_{12}^{j} ~Y_{22}^{j} ~Y_{23}^{j}\\ Y_{13}^{j} ~Y_{23}^{j} ~Y_{33}^{j} \end{array}\right) \succeq 0, ~j=1,\dots ,m,\nonumber \\ &{}~~~~~~~~~~~~~~~ \begin{aligned} &{}Y_{11}^{j}=\sigma _j-\left( 2\rho _j \theta _j^0 \omega _j+b_0^j \omega _j\right) \beta _j-\theta _j^0 \omega _j^2 \beta _j^2,\\ &{}Y_{12}^{j}=Y_{23}^{j}=0,\\ &{}2Y_{13}^{j}+Y_{22}^j=2\sigma _j-\left( 2\rho _j \theta _j^0 \omega _j+b_0^j \omega _j\right) \left( \beta _j+\gamma _j\right) -2 \theta _j^0 \omega _j^2 \beta _j\gamma _j,\\ &{}Y_{33}^{j}=\sigma _j-\left( 2\rho _j \theta _j^0 \omega _j+b_0^j \omega _j\right) \gamma _j-\theta _j^0 \omega _j^2 \gamma _j^2. \end{aligned} \end{aligned} \end{array} \right. } \end{aligned}$$

Corollary 3.4

Consider the problem \(\mathrm {(RTP)}\) with \(\mathcal {U}^i,\) \(i=1,\dots ,l,\) given by (14) and its reformulation problem \(\mathrm {(SDP)}\). Then,

$$\begin{aligned} \inf (\textrm{RTP})=\inf (\textrm{SDP}). \end{aligned}$$

Moreover, \((x,\rho ,W )\) is an optimal solution of the problem \(\mathrm {(RTP)}\) if and only if there exist \(\tau \in \mathbb {R},\) \(\delta \in \mathbb {R}^{lm},\) \(u \in \mathbb {R}^{pl},\) \(X \in \mathbb {R}^{3\times 3lm},\) \(\sigma \in \mathbb {R}^m\) and \(Y \in \mathbb {R}^{3\times 3\,m}\) such that \((x,\rho ,W,\tau ,\delta ,u,X,\sigma ,Y)\) is an optimal solution of the problem \(\mathrm {(SDP)}\).

Proof

Following similar arguments to those used in [14, Corollary 2.2], we can show that

$$\begin{aligned} \begin{aligned} A^i+\sum _{k=1}^p u^i_k A^i_k\succeq 0&\Longleftrightarrow \lambda _i + \sum _{k=1}^p u_k^i \ge 0 and \lambda _i - \sum _{k=1}^p u_k^i \ge 0\\&\Longleftrightarrow \sum _{k=1}^p |u_k^i|\le \lambda _i,~i=1,\dots ,l. \end{aligned} \end{aligned}$$
(15)

Then, the desired result is obtained.

Remark 3.4

Note that related results for SDP reformulations of robust optimization problems on polytopic uncertain sets have been investigated in [26, Lemma 1] and [7, Theorem 1]. However, Corollary 3.4 extends the results obtained in [26, Lemma 1] from static to the adjustable setting. Furthermore, Corollary 3.4 also extends the results obtained in [7, Theorem 1] from linear optimization models to quadratic optimization models.

Let us now consider the case when \(\mathcal {U}^i,\) \(i=1,\dots ,l,\) are boxes defined by

$$\begin{aligned} \mathcal {U}^i:=\left\{ u^i:=(u_1^i,\dots ,u_p^i)\in \mathbb {R}^p~\bigg |~|u_k^i|\le \lambda _i,~k=1,\dots ,p \right\} , ~i=1,\dots ,l, \end{aligned}$$
(16)

where \(\lambda _i>0,\) \(i=1,\dots ,l.\)

In this setting, the SDP reformulation of (RTP) becomes

$$\begin{aligned} { \left\{ \begin{array}{ll} \begin{aligned} &{}\inf \limits _{\begin{array}{c} x\in \mathbb {R}^n, \tau \in \mathbb {R}, \delta \in \mathbb {R}^{lm}, u\in \mathbb {R}^{pl}, \rho \in \mathbb {R}^m, \\ X\in \mathbb {R}^{3\times 3lm}, W\in \mathbb {R}^{m\times m}, \sigma \in \mathbb {R}^m, Y\in \mathbb {R}^{3\times 3m} \end{array}} x^{\top }Q_0x+q_0^{\top }x+\xi _0+\tau \nonumber \\ &{}~~~~~~~~s.t.~~~~\sum _{j=1}^m\delta _i^j\le -\left( \sum _{s=1}^n a_{s}^ix_s^2+\sum _{s=1}^n \left( q^{s}_i+\sum _{k=1}^p u_k^i q^{is}_k\right) x_s+\xi _i+\sum _{k=1}^p u_k^i \xi _k^i\right. \nonumber \\ &{}\quad \left. +\sum _{j=1}^m \theta _j^i \rho _j^2+\sum _{j=1}^m b_i^j \rho _j+t_i\right) ,~i=1,\dots ,l, \nonumber \\ &{}\quad \lambda _i + u_k^i\ge 0,~\lambda _i - u_k^i\ge 0, ~k=1,\dots ,p, ~i=1,\dots ,l, \nonumber \\ &{}~~~~~~~~~~~~~~~X^{ij}:= \left( \begin{array}{lll} X_{11}^{ij} ~X_{12}^{ij} ~X_{13}^{ij}\\ X_{12}^{ij} ~X_{22}^{ij} ~X_{23}^{ij}\\ X_{13}^{ij} ~X_{23}^{ij} ~X_{33}^{ij} \end{array}\right) \succeq 0, ~i=1,\dots ,l, ~j=1,\dots ,m,\nonumber \\ &{}~~~~~~~~~~~~~~ \begin{aligned} &{}X_{11}^{ij}=\delta _i^j-\left( 2\rho _j \theta _j^i \omega _j+b_i^j \omega _j\right) \beta _j-\theta _j^i \omega _j^2 \beta _j^2,\\ &{}X_{12}^{ij}=X_{23}^{ij}=0,\\ &{}2X_{13}^{ij}+X_{22}^{ij}=2\delta _i^j-\left( 2\rho _j \theta _j^i \omega _j+b_i^j \omega _j\right) \left( \beta _j+\gamma _j\right) -2 \theta _j^i \omega _j^2 \beta _j\gamma _j,\\ &{}X_{33}^{ij}=\delta _i^j-\left( 2\rho _j \theta _j^i \omega _j+b_i^j \omega _j\right) \gamma _j-\theta _j^i \omega _j^2 \gamma _j^2, \end{aligned} \nonumber \\ &{}~~~~~~~~~~~~~~\sum _{j=1}^m \sigma _j\le \tau -\sum _{j=1}^m \theta _j^0 \rho _j^2-\sum _{j=1}^m b_0^j \rho _j-t_0,\nonumber \\ &{}~~~~~~~~~~~~~~~Y^{j}:= \left( \begin{array}{lll} Y_{11}^{j} ~Y_{12}^{j} ~Y_{13}^{j}\\ Y_{12}^{j} ~Y_{22}^{j} ~Y_{23}^{j}\\ Y_{13}^{j} ~Y_{23}^{j} ~Y_{33}^{j} \end{array}\right) \succeq 0, ~j=1,\dots ,m,\nonumber \\ &{}~~~~~~~~~~~~~~ \begin{aligned} &{}Y_{11}^{j}=\sigma _j-\left( 2\rho _j \theta _j^0 \omega _j+b_0^j \omega _j\right) \beta _j-\theta _j^0 \omega _j^2 \beta _j^2,\\ &{}Y_{12}^{j}=Y_{23}^{j}=0,\\ &{}2Y_{13}^{j}+Y_{22}^j=2\sigma _j-\left( 2\rho _j \theta _j^0 \omega _j+b_0^j \omega _j\right) \left( \beta _j+\gamma _j\right) -2 \theta _j^0 \omega _j^2 \beta _j\gamma _j,\\ &{}Y_{33}^{j}=\sigma _j-\left( 2\rho _j \theta _j^0 \omega _j+b_0^j \omega _j\right) \gamma _j-\theta _j^0 \omega _j^2 \gamma _j^2. \end{aligned} \end{aligned} \end{array} \right. } \end{aligned}$$

Corollary 3.5

Consider the problem \(\mathrm {(RTP)}\) with \(\mathcal {U}^i,\) \(i=1,\dots ,l,\) given by (16) and its reformulation problem \(\mathrm {(SDP)}\). Then,

$$\begin{aligned} \inf (\textrm{RTP})=\inf (\textrm{SDP}). \end{aligned}$$

Moreover, \((x,\rho ,W )\) is an optimal solution of the problem \(\mathrm {(RTP)}\) if and only if there exist \(\tau \in \mathbb {R},\) \(\delta \in \mathbb {R}^{lm},\) \(u \in \mathbb {R}^{pl},\) \(X \in \mathbb {R}^{3\times 3lm},\) \(\sigma \in \mathbb {R}^m\) and \(Y \in \mathbb {R}^{3\times 3\,m}\) such that \((x,\rho ,W,\tau ,\delta ,u,X,\sigma ,Y)\) is an optimal solution of the problem \(\mathrm {(SDP)}\).

Proof

Using similar arguments to those used in [14, Corollary 2.2], we can show that

$$\begin{aligned} A^i+\sum _{k=1}^p u^i_k A^i_k\succeq 0 \Longleftrightarrow \lambda _i + u_k^i \ge 0 and \lambda _i -u_k^i \ge 0 \Longleftrightarrow |u_k^i|\le \lambda _i,~i=1,\dots ,l.\nonumber \\ \end{aligned}$$
(17)

Then, we obtain the desired result.

4 Applications

In this section, we apply the results obtained in the previous sections to fractionally adjustable robust optimization problems.

In what follows, suppose that \(g_0,\) \(f_i\), \(g_i,\) \(y(\cdot )\), \(\mathcal {U}^i\) and \(\mathcal {U}_{box}\) are considered as before. Let \(Q_0\), \(\widetilde{Q}_0 \in \mathbb {S}^{n},\) \(q_0\), \(\widetilde{q}_0\in \mathbb {R}^n\), and \(\xi _0\), \(\widetilde{\xi }_0\in \mathbb {R}.\) We consider the following fractionally adjustable robust quadratic optimization

$$\begin{aligned} (\text{ FTP})~~~\left\{ \begin{array}{ll} \mathop {\text{ inf }}\limits _{\begin{array}{c} x\in \mathbb {R}^{n}, \rho \in \mathbb {R}^m,\\ W\in \mathbb {R}^{m\times m} \end{array}} \frac{x^{\top }{Q}_0x+{q}_0^{\top }x+{\xi }_0}{x^{\top }\widetilde{Q}_0x+\widetilde{q}_0^{\top }x+\widetilde{\xi }_0}+\frac{\mathop {\text{ max }}\limits _{z\in \mathcal {U}_{box}} g_0(y(z))}{\mathop {\text{ min }}\limits _{z\in \mathcal {U}_{box}} \widetilde{g}_0(y(z))}\\ ~~~~~s.t.~~~~f_i(x,u^i)+g_i(y(z))\le 0, ~\forall u^i\in \mathcal {U}^i,~ i=1,\dots ,l,\\ ~~~~~~~~~~~~~y(z)=\rho +W z, ~\forall z\in \mathcal {U}_{box}, \end{array} \right. \end{aligned}$$

where \(\widetilde{g}_0(y(z)):=(y(z))^{\top }\widetilde{B}_0y(z)+\widetilde{b}_0^{\top }y(z)+\widetilde{t_0}\) with \(\widetilde{B}_0:=diag (\widetilde{\theta }_1^0,\dots ,\widetilde{\theta }_m^0)\in \mathbb {R}^{m\times m},\) \(\widetilde{b}_0:=(\widetilde{b}_0^1,\dots ,\widetilde{b}_0^m)\in \mathbb {R}^m\) and \(\widetilde{t}_0\in \mathbb {R}.\) Moreover, we assume that \(x^{\top }\widetilde{Q}_0x+\widetilde{q}_0^{\top }x+\widetilde{\xi }_0>0\) and \(\widetilde{g}_0(y(z))>0\) are in the feasible set of (FTP).

The SDP reformulation of (FTP) is given as follows:

$$\begin{aligned} { (\text{ FSDP})~~~\left\{ \begin{array}{ll} \begin{aligned} &{}\inf \limits _{\begin{array}{c} x\in \mathbb {R}^n, \tau \in \mathbb {R}, \delta \in \mathbb {R}^{lm}, u\in \mathbb {R}^{pl}, \rho \in \mathbb {R}^m, \\ X\in \mathbb {R}^{3\times 3lm}, W\in \mathbb {R}^{m\times m}, \sigma \in \mathbb {R}^m, Y\in \mathbb {R}^{3\times 3m} \end{array}} ~\frac{x^{\top }Q_0x+q_0^{\top }x+\xi _0}{x^{\top }\widetilde{Q}_0x+\widetilde{q}_0^{\top }x+\widetilde{\xi }_0}+\tau \nonumber \\ &{}~~~~~~~~s.t.~~~~\sum _{j=1}^m\delta _i^j\le -\left( \sum _{s=1}^n a_{s}^ix_s^2+\sum _{s=1}^n \left( q^{s}_i+\sum _{k=1}^p u_k^i q^{is}_k\right) x_s+\xi _i+\sum _{k=1}^p u_k^i \xi _k^i\right. \nonumber \\ &{}~~~~~~~~~~~~~~~\left. +\sum _{j=1}^m \theta _j^i \rho _j^2+\sum _{j=1}^m b_i^j \rho _j+t_i\right) ,~i=1,\dots ,l, \nonumber \\ &{}~~~~~~~~~~~~~~~A^i+\sum _{k=1}^p u^i_k A^i_k\succeq 0,~i=1,\dots ,l,\nonumber \\ &{}~~~~~~~~~~~~~~~X^{ij}:= \left( \begin{array}{lll} X_{11}^{ij} ~X_{12}^{ij} ~X_{13}^{ij}\\ X_{12}^{ij} ~X_{22}^{ij} ~X_{23}^{ij}\\ X_{13}^{ij} ~X_{23}^{ij} ~X_{33}^{ij} \end{array}\right) \succeq 0, ~i=1,\dots ,l, ~j=1,\dots ,m,\nonumber \\ &{}~~~~~~~~~~~~~~ \begin{aligned} &{}X_{11}^{ij}=\delta _i^j-\left( 2\rho _j \theta _j^i \omega _j+b_i^j \omega _j\right) \beta _j-\theta _j^i \omega _j^2 \beta _j^2,\\ &{}X_{12}^{ij}=X_{23}^{ij}=0,\\ &{}2X_{13}^{ij}+X_{22}^{ij}=2\delta _i^j-\left( 2\rho _j \theta _j^i \omega _j+b_i^j \omega _j\right) \left( \beta _j+\gamma _j\right) -2 \theta _j^i \omega _j^2 \beta _j \gamma _j,\\ &{}X_{33}^{ij}=\delta _i^j-\left( 2\rho _j \theta _j^i \omega _j+b_i^j \omega _j\right) \gamma _j-\theta _j^i \omega _j^2 \gamma _j^2, \end{aligned} \nonumber \\ &{}~~~~~~~~~~~~~~\sum _{j=1}^m \sigma _j\le \tau -\sum _{j=1}^m \left( \theta _j^0-\tau \widetilde{\theta }_j^0\right) \rho _j^2-\sum _{j=1}^m \left( b_0^j -\tau \widetilde{b}_0^j \right) \rho _j-\left( t_0-\tau \widetilde{t}_0\right) ,\nonumber \\ &{}~~~~~~~~~~~~~~~Y^{j}:= \left( \begin{array}{lll} Y_{11}^{j} ~Y_{12}^{j} ~Y_{13}^{j}\\ Y_{12}^{j} ~Y_{22}^{j} ~Y_{23}^{j}\\ Y_{13}^{j} ~Y_{23}^{j} ~Y_{33}^{j} \end{array}\right) \succeq 0, ~j=1,\dots ,m,\nonumber \\ &{}~~~~~~~~~~~~~~ \begin{aligned} &{}Y_{11}^{j}=\sigma _j-\left( 2 \left( \theta _j^0-\tau \widetilde{\theta }_j^0\right) \rho _j \omega _j+\left( b_0^j-\tau \widetilde{b}_0^j\right) \omega _j\right) \beta _j-\left( \theta _j^0-\tau \widetilde{\theta }_j^0\right) \omega _j^2 \beta _j^2,\\ &{}Y_{12}^{j}=Y_{23}^{j}=0,\\ &{}2Y_{13}^{j}+Y_{22}^j=2\sigma _j-\left( 2 \left( \theta _j^0-\tau \widetilde{\theta }_j^0\right) \rho _j \omega _j+\left( b_0^j-\tau \widetilde{b}_0^j\right) \omega _j\right) \left( \beta _j+\gamma _j\right) \\ &{}~~~~~~~~~~~~~~~~~-2 \left( \theta _j^0-\tau \widetilde{\theta }_j^0\right) \omega _j^2 \beta _j\gamma _j,\\ &{}Y_{33}^{j}=\sigma _j-\left( 2\left( \theta _j^0-\tau \widetilde{\theta }_j^0\right) \rho _j \omega _j+\left( b_0^j-\tau \widetilde{b}_0^j\right) \omega _j\right) \gamma _j-\left( \theta _j^0-\tau \widetilde{\theta }_j^0\right) \omega _j^2 \gamma _j^2. \end{aligned} \end{aligned} \end{array} \right. } \end{aligned}$$

Now, we give the following theorem which describes an exact SDP reformulation for (FTP).

Theorem 4.1

Consider the problem \(\mathrm {(FTP)}\) with \(\mathcal {U}^i,\) \(i=1,\dots ,l,\) given by (1) and its reformulation problem \(\mathrm {(FSDP)}\). Then,

$$\begin{aligned} \inf (\textrm{FTP})=\inf (\textrm{FSDP}). \end{aligned}$$

Moreover, \((x,\rho ,W)\) is an optimal solution of the problem \(\mathrm {(FTP)}\) if and only if there exist \(\tau \in \mathbb {R},\) \(\delta \in \mathbb {R}^{lm},\) \(u \in \mathbb {R}^{pl},\) \(X \in \mathbb {R}^{3\times 3lm},\) \(\sigma \in \mathbb {R}^m\) and \(Y \in \mathbb {R}^{3\times 3\,m}\) such that \((x,\rho ,W,\tau ,\delta ,u,X,\sigma ,Y)\) is an optimal solution of the problem \(\mathrm {(FSDP) }\).

Proof

Obviously, (FTP) is equivalent to

$$\begin{aligned} \left\{ \begin{array}{ll} \mathop {\text{ inf }}\limits _{\begin{array}{c} x\in \mathbb {R}^{n}, \tau \in \mathbb {R}, \\ \rho \in \mathbb {R}^m, W\in \mathbb {R}^{m\times m} \end{array}} \frac{x^{\top }{Q}_0x+{q}_0^{\top }x+{\xi }_0}{x^{\top }\widetilde{Q}_0x+\widetilde{q}_0^{\top }x+\widetilde{\xi }_0}+\tau \\ ~~~~~~~s.t.~~~~~~~f_i(x,u^i)+g_i(y(z))\le 0, ~\forall u^i\in \mathcal {U}^i,~ i=1,\dots ,l,\\ ~~~~~~~~~~~~~~~~~\frac{\mathop {\text{ max }}\limits _{z\in \mathcal {U}_{box}} g_0(y(z))}{\mathop {\text{ min }}\limits _{z\in \mathcal {U}_{box}} \widetilde{g}_0(y(z))}\le \tau , \\ ~~~~~~~~~~~~~~~~~~y(z)=\rho +W z, ~\forall z\in \mathcal {U}_{box}. \end{array}\right. \end{aligned}$$
(18)

The problem (18) is equivalent to

$$\begin{aligned} \left\{ \begin{array}{ll} \mathop {\text{ inf }}\limits _{\begin{array}{c} x\in \mathbb {R}^{n}, \tau \in \mathbb {R}, \\ \rho \in \mathbb {R}^m, W\in \mathbb {R}^{m\times m} \end{array}} \frac{x^{\top }{Q}_0x+{q}_0^{\top }x+{\xi }_0}{x^{\top }\widetilde{Q}_0x+\widetilde{q}_0^{\top }x+\widetilde{\xi }_0}+\tau \nonumber \\ ~~~~~~~s.t.~~~~~~~f_i(x,u^i)+g_i(y(z))\le 0, ~\forall u^i\in \mathcal {U}^i,~ i=1,\dots ,l,\nonumber \\ ~~~~~~~~~~~~~~~~~~g_0(y(z))- \tau \widetilde{g}_0(y(z))\le 0,\nonumber \\ ~~~~~~~~~~~~~~~~~~y(z)=\rho +W z, ~\forall z\in \mathcal {U}_{box}, \end{array}\right. \end{aligned}$$

which can be written as:

$$\begin{aligned} \left\{ \begin{array}{ll} \mathop {\text{ inf }}\limits _{\begin{array}{c} x\in \mathbb {R}^{n}, \tau \in \mathbb {R}, u\in \mathbb {R}^{pl},\\ \rho \in \mathbb {R}^m, W\in \mathbb {R}^{m\times m} \end{array}} ~\frac{x^{\top }Q_0x+q_0^{\top }x+\xi _0}{x^{\top }\widetilde{Q}_0x+\widetilde{q}_0^{\top }x+\widetilde{\xi }_0}+\tau \\ ~~~~~~~~s.t.~~~~~x^{\top }Q_ix+q_i^{\top }x+\xi _i+\sum _{k=1}^p u^i_k\left( (q^i_k)^{\top }x+\xi ^i_k\right) +(y(z))^{\top }B_iy(z)\nonumber \\ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+b_i^{\top }y(z)+t_i\le 0, ~i=1,\dots ,l,\\ ~~~~~~~~~~~~~~~~~A^i+\sum _{k=1}^p u^i_k A^i_k\succeq 0,~i=1,\dots ,l,\nonumber \\ ~~~~~~~~~~~~~~~~~(y(z))^{\top }B_0y(z)+b_0^{\top }y(z)+t_0- \tau \left( (y(z))^{\top }\widetilde{B}_0y(z)+\widetilde{b}_0^{\top }y(z)+\widetilde{t}_0\right) \le 0,\\ ~~~~~~~~~~~~~~~~~y(z)=\rho +W z, ~\forall z\in \mathcal {U}_{box}. \end{array}\right. \end{aligned}$$

Now, using similar arguments as in the proof of Theorem 3.1, it is easy to show that Theorem 4.1 holds.

Remark 4.1

Note that related results for SDP reformulations of fractional optimization problems have been investigated in [22, Theorem 3.2] and [20, Theorem 3.5]. However, Theorem 4.1 extends [22, Theorem 3.2] from deterministic models to the uncertain setting. Theorem 4.1 also extends the results obtained in [20, Theorem 3.5] from static to the adjustable setting.

In the special case that \(B_0=\text{ O}_{m\times m},\) \(b_0=0_m\) and \(t_0=0\), (FTP) collapses to

$$\begin{aligned} (\text{ FTP}_0)~~~\left\{ \begin{array}{ll}\mathop {\text{ inf }}\limits _{\begin{array}{c} x\in \mathbb {R}^{n}, \rho \in \mathbb {R}^m, \\ W\in \mathbb {R}^{m\times m} \end{array}} \frac{x^{\top }Q_0x+q_0^{\top }x+\xi _0}{x^{\top }\widetilde{Q}_0x+\widetilde{q}_0^{\top }x+\widetilde{\xi }_0}\\ ~~~~s.t.~~~~~~x^{\top }Q_ix+q_i^{\top }x+\xi _i+\sum _{k=1}^p u^i_k\left( (q^i_k)^{\top }x+\xi ^i_k\right) +(y(z))^{\top }B_iy(z)\nonumber \\ ~~~~~~~~~~~~~+b_i^{\top }y(z)+t_i\le 0, ~\forall (u_1^i,\dots ,u_p^i)\in \mathcal {U}^i,~i=1,\dots ,l,\nonumber \\ ~~~~~~~~~~~~~~y(z)=\rho +W z, ~\forall z\in \mathcal {U}_{box}, \end{array} \right. \end{aligned}$$

and (FSDP) becomes

$$\begin{aligned} (\text{ FSDP}_0)~~~ \left\{ \begin{array}{ll} \begin{aligned} &{}\inf \limits _{\begin{array}{c} x\in \mathbb {R}^n, \delta \in \mathbb {R}^{lm}, u\in \mathbb {R}^{pl}, \\ \rho \in \mathbb {R}^m, X\in \mathbb {R}^{3\times 3lm}, W\in \mathbb {R}^{m\times m} \end{array}} ~\frac{x^{\top }Q_0x+q_0^{\top }x+\xi _0}{x^{\top }\widetilde{Q}_0x+\widetilde{q}_0^{\top }x+\widetilde{\xi }_0}\nonumber \\ &{}~~~~~~~~s.t.~~~~\sum _{j=1}^m\delta _i^j\le -\left( \sum _{s=1}^n a_{s}^ix_s^2+\sum _{s=1}^n \left( q^{s}_i+\sum _{k=1}^p u_k^i q^{is}_k\right) x_s+\xi _i+\sum _{k=1}^p u_k^i \xi _k^i\right. \nonumber \\ &{}~~~~~~~~~~~~~~~\left. +\sum _{j=1}^m \theta _j^i \rho _j^2+\sum _{j=1}^m b_i^j \rho _j+t_i\right) ,~i=1,\dots ,l, \nonumber \\ &{}~~~~~~~~~~~~~~~A^i+\sum _{k=1}^p u^i_k A^i_k\succeq 0,~i=1,\dots ,l,\nonumber \\ &{}~~~~~~~~~~~~~~~X^{ij}:= \left( \begin{array}{lll} X_{11}^{ij} ~X_{12}^{ij} ~X_{13}^{ij}\\ X_{12}^{ij} ~X_{22}^{ij} ~X_{23}^{ij}\\ X_{13}^{ij} ~X_{23}^{ij} ~X_{33}^{ij} \end{array}\right) \succeq 0, ~i=1,\dots ,l, ~j=1,\dots ,m,\nonumber \\ &{}~~~~~~~~~~~~~~ \begin{aligned} &{}X_{11}^{ij}=\delta _i^j-\left( 2\rho _j \theta _j^i \omega _j+b_i^j \omega _j\right) \beta _j-\theta _j^i \omega _j^2 \beta _j^2,\\ &{}X_{12}^{ij}=X_{23}^{ij}=0,\\ &{}2X_{13}^{ij}+X_{22}^{ij}=2\delta _i^j-\left( 2\rho _j \theta _j^i \omega _j+b_i^j \omega _j\right) \left( \beta _j+\gamma _j\right) -2 \theta _j^i \omega _j^2 \beta _j\gamma _j,\\ &{}X_{33}^{ij}=\delta _i^j-\left( 2\rho _j \theta _j^i \omega _j+b_i^j \omega _j\right) \gamma _j-\theta _j^i \omega _j^2 \gamma _j^2. \end{aligned} \end{aligned} \end{array} \right. \end{aligned}$$

By virtue of Theorem 4.1, we can easily obtain the following result.

Corollary 4.1

Consider the problem \(\mathrm {(FTP_0)}\) with \(\mathcal {U}^i,\) \(i=1,\dots ,l,\) given by (1) and its reformulation problem \(\mathrm {(FSDP_0)}\). Then,

$$\begin{aligned} \inf (\textrm{FTP}_0)=\inf (\textrm{FSDP}_0). \end{aligned}$$

Moreover, \((x,\rho ,W )\) is an optimal solution of the problem \(\mathrm {(FTP_0)}\) if and only if there exist \(\delta \in \mathbb {R}^{lm},\) \(u \in \mathbb {R}^{pl}\) and \(X \in \mathbb {R}^{3\times 3lm}\) such that \((x,\rho ,W, \delta ,u,X)\) is an optimal solution of the problem \(\mathrm {(FSDP_0)}\).

In the special case when (FTP) with \(t_0=\widetilde{t}_0=0\) and \(Q_i=\widetilde{Q}_0=B_i=\widetilde{B}_0=\text{ O}_{m\times m},\) \(i=0,1,\dots ,l,\) (FTP) collapses to the following fractionally adjustable robust linear optimization

$$\begin{aligned} (\text{ FTP}_1)~~~\left\{ \begin{array}{ll} \mathop {\text{ inf }}\limits _{\begin{array}{c} x\in \mathbb {R}^{n}, \rho \in \mathbb {R}^m, \\ W\in \mathbb {R}^{m\times m} \end{array}} \frac{q_0^{\top }x+\xi _0}{\widetilde{q}_0^{\top }x+\widetilde{\xi }_0}+\frac{\mathop {\text{ max }}\limits _{z\in \mathcal {U}_{box}} b_0^{\top }y(z)}{\mathop {\text{ min }}\limits _{z\in \mathcal {U}_{box}} \widetilde{b}_0^{\top }y(z)}\\ ~~~s.t.~~~~q_i^{\top }x+\xi _i+\sum _{k=1}^p u^i_k\left( (q^i_k)^{\top }x+\xi ^i_k\right) +b_i^{\top }y(z)\\ ~~~~~~~~~~~~+t_i\le 0, ~\forall (u_1^i,\dots ,u_p^i)\in \mathcal {U}^i,~ i=1,\dots ,l,\\ ~~~~~~~~~~~~~y(z)=\rho +W z, ~\forall z\in \mathcal {U}_{box}, \end{array} ~~~~~~~~~~~~~~~~~~~~~~\right. \end{aligned}$$

and (FSDP) becomes

$$\begin{aligned} (\text{ FSDP}_1)~~~\left\{ \begin{array}{ll} \begin{aligned} &{}\inf \limits _{\begin{array}{c} x\in \mathbb {R}^n, \tau \in \mathbb {R}, \delta \in \mathbb {R}^{lm}, u\in \mathbb {R}^{pl}, \rho \in \mathbb {R}^m, \\ X\in \mathbb {R}^{3\times 3lm}, W\in \mathbb {R}^{m\times m}, \sigma \in \mathbb {R}^m, Y\in \mathbb {R}^{3\times 3m} \end{array}} ~\frac{q_0^{\top }x+\xi _0}{\widetilde{q}_0^{\top }x+\widetilde{\xi }_0}+\tau \nonumber \\ &{}~~~~~~~~s.t.~~~~\sum _{j=1}^m\delta _i^j\le -\left( \sum _{s=1}^n \left( q^{s}_i+\sum _{k=1}^p u_k^i q^{is}_k\right) x_s+\xi _i+\sum _{k=1}^p u_k^i \xi _k^i\right. \nonumber \\ &{}~~~~~~~~~~~~~~~\left. +\sum _{j=1}^m b_i^j \rho _j+t_i\right) ,~i=1,\dots ,l, \nonumber \\ &{}~~~~~~~~~~~~~~~A^i+\sum _{k=1}^p u^i_k A^i_k\succeq 0,~i=1,\dots ,l,\nonumber \\ &{}~~~~~~~~~~~~~~~X^{ij}:= \left( \begin{array}{lll} X_{11}^{ij} ~X_{12}^{ij} ~X_{13}^{ij}\\ X_{12}^{ij} ~X_{22}^{ij} ~X_{23}^{ij}\\ X_{13}^{ij} ~X_{23}^{ij} ~X_{33}^{ij} \end{array}\right) \succeq 0, ~i=1,\dots ,l, ~j=1,\dots ,m,\nonumber \\ &{}~~~~~~~~~~~~~~ \begin{aligned} &{}X_{11}^{ij}=\delta _i^j-b_i^j \omega _j \beta _j,\\ &{}X_{12}^{ij}=X_{23}^{ij}=0,\\ &{}2X_{13}^{ij}+X_{22}^{ij}=2\delta _i^j-b_i^j \omega _j \left( \beta _j+\gamma _j\right) ,\\ &{}X_{33}^{ij}=\delta _i^j-b_i^j \omega _j \gamma _j, \end{aligned} \nonumber \\ &{}~~~~~~~~~~~~~~\sum _{j=1}^m \sigma _j\le \tau -\sum _{j=1}^m \left( b_0^j-\tau \widetilde{b}_0^j \right) \rho _j,\nonumber \\ &{}~~~~~~~~~~~~~~~Y^{j}:= \left( \begin{array}{lll} Y_{11}^{j} ~Y_{12}^{j} ~Y_{13}^{j}\\ Y_{12}^{j} ~Y_{22}^{j} ~Y_{23}^{j}\\ Y_{13}^{j} ~Y_{23}^{j} ~Y_{33}^{j} \end{array}\right) \succeq 0, ~j=1,\dots ,m,\nonumber \\ &{}~~~~~~~~~~~~~~ \begin{aligned} &{}Y_{11}^{j}=\sigma _j-\left( b_0^j-\tau \widetilde{b}_0^j\right) \omega _j \beta _j,\\ &{}Y_{12}^{j}=Y_{23}^{j}=0,\\ &{}2Y_{13}^{j}+Y_{22}^j=2\sigma _j-\left( b_0^j-\tau \widetilde{b}_0^j\right) \omega _j \left( \beta _j+\gamma _j\right) \\ &{}Y_{33}^{j}=\sigma _j-\left( b_0^j-\tau \widetilde{b}_0^j\right) \omega _j \gamma _j. \end{aligned} \end{aligned} \end{array} \right. \end{aligned}$$

Corollary 4.2

Consider the problem \(\mathrm {(FTP_1)}\) with \(\mathcal {U}^i,\) \(i=1,\dots ,l,\) given by (1) and its reformulation problem \(\mathrm {(FSDP_1)}\). Then,

$$\begin{aligned} \inf (\textrm{FTP}_1)=\inf (\textrm{FSDP}_1). \end{aligned}$$

Moreover, \((x,\rho ,W )\) is an optimal solution of the problem \(\mathrm {(FTP_1)}\) if and only if there exist \(\tau \in \mathbb {R},\) \(\delta \in \mathbb {R}^{lm},\) \(u \in \mathbb {R}^{pl},\) \(X \in \mathbb {R}^{3\times 3lm},\) \(\sigma \in \mathbb {R}^m\) and \(Y \in \mathbb {R}^{3\times 3\,m}\) such that \((x,\rho ,W,\tau ,\delta ,u,X,\sigma ,Y)\) is an optimal solution of the problem \(\mathrm {(FSDP_1)}\).

At the end of this section, we examine the category of uncertain sets such that (FTP) has the exact SDP reformulation. We first consider the case when \(\mathcal {U}^i,\) \(i=1,\dots ,l,\) are ellipsoids in (12). In this case, its SDP reformulation becomes

$$\begin{aligned} \left\{ \begin{array}{ll} \begin{aligned} &{}\inf \limits _{\begin{array}{c} x\in \mathbb {R}^n, \tau \in \mathbb {R}, \delta \in \mathbb {R}^{lm}, u\in \mathbb {R}^{pl}, \rho \in \mathbb {R}^m, \\ X\in \mathbb {R}^{3\times 3lm}, W\in \mathbb {R}^{m\times m}, \sigma \in \mathbb {R}^m, Y\in \mathbb {R}^{3\times 3m} \end{array}} ~\frac{x^{\top }Q_0x+q_0^{\top }x+\xi _0}{x^{\top }\widetilde{Q}_0x+\widetilde{q}_0^{\top }x+\widetilde{\xi }_0}+\tau \nonumber \\ &{}~~~~~~~~s.t.~~~~\sum _{j=1}^m\delta _i^j\le -\left( \sum _{s=1}^n a_{s}^ix_s^2+\sum _{s=1}^n \left( q^{s}_i+\sum _{k=1}^p u_k^i q^{is}_k\right) x_s+\xi _i+\sum _{k=1}^p u_k^i \xi _k^i\right. \nonumber \\ &{}~~~~~~~~~~~~~~~\left. +\sum _{j=1}^m \theta _j^i \rho _j^2+\sum _{j=1}^m b_i^j \rho _j+t_i\right) ,~i=1,\dots ,l, \nonumber \\ &{}~~~~~~~~~~~~~~~\left( \begin{array}{lll} (E^i)^{-1} &{}u^i\\ (u^i)^{\top } &{}1 \end{array}\right) \succeq 0, ~i=1,\dots ,l,\nonumber \\ &{}~~~~~~~~~~~~~~~X^{ij}:= \left( \begin{array}{lll} X_{11}^{ij} ~X_{12}^{ij} ~X_{13}^{ij}\\ X_{12}^{ij} ~X_{22}^{ij} ~X_{23}^{ij}\\ X_{13}^{ij} ~X_{23}^{ij} ~X_{33}^{ij} \end{array}\right) \succeq 0, ~i=1,\dots ,l, ~j=1,\dots ,m,\nonumber \\ &{}~~~~~~~~~~~~~~ \begin{aligned} &{}X_{11}^{ij}=\delta _i^j-\left( 2\rho _j \theta _j^i \omega _j+b_i^j \omega _j\right) \beta _j-\theta _j^i \omega _j^2 \beta _j^2,\\ &{}X_{12}^{ij}=X_{23}^{ij}=0,\\ &{}2X_{13}^{ij}+X_{22}^{ij}=2\delta _i^j-\left( 2\rho _j \theta _j^i \omega _j+b_i^j \omega _j\right) \left( \beta _j+\gamma _j\right) -2 \theta _j^i \omega _j^2 \beta _j\gamma _j,\\ &{}X_{33}^{ij}=\delta _i^j-\left( 2\rho _j \theta _j^i \omega _j+b_i^j \omega _j\right) \gamma _j-\theta _j^i \omega _j^2 \gamma _j^2, \end{aligned} \nonumber \\ &{}~~~~~~~~~~~~~~\sum _{j=1}^m \sigma _j\le \tau -\sum _{j=1}^m \left( \theta _j^0-\tau \widetilde{\theta }_j^0\right) \rho _j^2-\sum _{j=1}^m \left( b_0^j -\tau \widetilde{b}_0^j \right) \rho _j-\left( t_0-\tau \widetilde{t}_0\right) ,\nonumber \\ &{}~~~~~~~~~~~~~~~Y^{j}:= \left( \begin{array}{lll} Y_{11}^{j} ~Y_{12}^{j} ~Y_{13}^{j}\\ Y_{12}^{j} ~Y_{22}^{j} ~Y_{23}^{j}\\ Y_{13}^{j} ~Y_{23}^{j} ~Y_{33}^{j} \end{array}\right) \succeq 0, ~j=1,\dots ,m,\nonumber \\ &{}~~~~~~~~~~~~~~ \begin{aligned} &{}Y_{11}^{j}=\sigma _j-\left( 2 \left( \theta _j^0-\tau \widetilde{\theta }_j^0\right) \rho _j \omega _j+\left( b_0^j-\tau \widetilde{b}_0^j\right) \omega _j\right) \beta _j-\left( \theta _j^0-\tau \widetilde{\theta }_j^0\right) \omega _j^2 \beta _j^2,\\ &{}Y_{12}^{j}=Y_{23}^{j}=0,\\ &{}2Y_{13}^{j}+Y_{22}^j=2\sigma _j-\left( 2 \left( \theta _j^0-\tau \widetilde{\theta }_j^0\right) \rho _j \omega _j+\left( b_0^j-\tau \widetilde{b}_0^j\right) \omega _j\right) \left( \beta _j+\gamma _j\right) \\ &{}~~~~~~~~~~~~~~~~~-2 \left( \theta _j^0-\tau \widetilde{\theta }_j^0\right) \omega _j^2 \beta _j\gamma _j,\\ &{}Y_{33}^{j}=\sigma _j-\left( 2\left( \theta _j^0-\tau \widetilde{\theta }_j^0\right) \rho _j \omega _j+\left( b_0^j-\tau \widetilde{b}_0^j\right) \omega _j\right) \gamma _j-\left( \theta _j^0-\tau \widetilde{\theta }_j^0\right) \omega _j^2 \gamma _j^2. \end{aligned} \end{aligned} \end{array}\right. \end{aligned}$$

Corollary 4.3

Consider the problem \(\mathrm {(FTP)}\) with \(\mathcal {U}^i,\) \(i=1,\dots ,l,\) given by (12) and its reformulation problem \(\mathrm {(FSDP)}\). Then,

$$\begin{aligned} \inf (\textrm{FTP})=\inf \mathrm {(FSDP)}. \end{aligned}$$

Moreover, \((x,\rho ,W)\) is an optimal solution of the problem \(\mathrm {(FTP)}\) if and only if there exist \(\tau \in \mathbb {R},\) \(\delta \in \mathbb {R}^{lm},\) \(u \in \mathbb {R}^{pl},\) \(X \in \mathbb {R}^{3\times 3lm},\) \(\sigma \in \mathbb {R}^m\) and \(Y \in \mathbb {R}^{3\times 3\,m}\) such that \((x,\rho ,W,\tau ,\delta ,u,X,\sigma ,Y)\) is an optimal solution of the problem \(\mathrm {(FSDP)}\).

Proof

Together with (13) and Theorem 4.1, we can easily get the desired result.

We now consider the case where \(\mathcal {U}^i,\) \(i=1,\dots ,l,\) are cross-polytopes in (14). In this case, the SDP reformulation of (FTP) becomes

$$\begin{aligned} \left\{ \begin{array}{ll} \begin{aligned} &{}\inf \limits _{\begin{array}{c} x\in \mathbb {R}^n, \tau \in \mathbb {R}, \delta \in \mathbb {R}^{lm}, u\in \mathbb {R}^{pl}, \rho \in \mathbb {R}^m, \\ X\in \mathbb {R}^{3\times 3lm}, W\in \mathbb {R}^{m\times m}, \sigma \in \mathbb {R}^m, Y\in \mathbb {R}^{3\times 3m} \end{array}} ~\frac{x^{\top }Q_0x+q_0^{\top }x+\xi _0}{x^{\top }\widetilde{Q}_0x+\widetilde{q}_0^{\top }x+\widetilde{\xi }_0}+\tau \nonumber \\ &{}~~~~~~~~s.t.~~~~\sum _{j=1}^m\delta _i^j\le -\left( \sum _{s=1}^n a_{s}^ix_s^2+\sum _{s=1}^n \left( q^{s}_i+\sum _{k=1}^p u_k^i q^{is}_k\right) x_s+\xi _i+\sum _{k=1}^p u_k^i \xi _k^i\right. \nonumber \\ &{}~~~~~~~~~~~~~~~\left. +\sum _{j=1}^m \theta _j^i \rho _j^2+\sum _{j=1}^m b_i^j \rho _j+t_i\right) ,~i=1,\dots ,l, \nonumber \\ &{}~~~~~~~~~~~~~~~\lambda _i+\sum _{k=1}^p u_k^i\ge 0,~\lambda _i-\sum _{k=1}^p u_k^i\ge 0,~i=1,\dots ,l,\nonumber \\ &{}~~~~~~~~~~~~~~~X^{ij}:= \left( \begin{array}{lll} X_{11}^{ij} ~X_{12}^{ij} ~X_{13}^{ij}\\ X_{12}^{ij} ~X_{22}^{ij} ~X_{23}^{ij}\\ X_{13}^{ij} ~X_{23}^{ij} ~X_{33}^{ij} \end{array}\right) \succeq 0, ~i=1,\dots ,l, ~j=1,\dots ,m,\nonumber \\ &{}~~~~~~~~~~~~~~ \begin{aligned} &{}X_{11}^{ij}=\delta _i^j-\left( 2\rho _j \theta _j^i \omega _j+b_i^j \omega _j\right) \beta _j-\theta _j^i \omega _j^2 \beta _j^2,\\ &{}X_{12}^{ij}=X_{23}^{ij}=0,\\ &{}2X_{13}^{ij}+X_{22}^{ij}=2\delta _i^j-\left( 2\rho _j \theta _j^i \omega _j+b_i^j \omega _j\right) \left( \beta _j+\gamma _j\right) -2 \theta _j^i \omega _j^2 \beta _j\gamma _j,\\ &{}X_{33}^{ij}=\delta _i^j-\left( 2\rho _j \theta _j^i \omega _j+b_i^j \omega _j\right) \gamma _j-\theta _j^i \omega _j^2 \gamma _j^2, \end{aligned} \nonumber \\ &{}~~~~~~~~~~~~~~\sum _{j=1}^m \sigma _j\le \tau -\sum _{j=1}^m \left( \theta _j^0-\tau \widetilde{\theta }_j^0\right) \rho _j^2-\sum _{j=1}^m \left( b_0^j -\tau \widetilde{b}_0^j \right) \rho _j-\left( t_0-\tau \widetilde{t}_0\right) ,\nonumber \\ &{}~~~~~~~~~~~~~~~Y^{j}:= \left( \begin{array}{lll} Y_{11}^{j} ~Y_{12}^{j} ~Y_{13}^{j}\\ Y_{12}^{j} ~Y_{22}^{j} ~Y_{23}^{j}\\ Y_{13}^{j} ~Y_{23}^{j} ~Y_{33}^{j} \end{array}\right) \succeq 0, ~j=1,\dots ,m,\nonumber \\ &{}~~~~~~~~~~~~~~ \begin{aligned} &{}Y_{11}^{j}=\sigma _j-\left( 2 \left( \theta _j^0-\tau \widetilde{\theta }_j^0\right) \rho _j \omega _j+\left( b_0^j-\tau \widetilde{b}_0^j\right) \omega _j\right) \beta _j-\left( \theta _j^0-\tau \widetilde{\theta }_j^0\right) \omega _j^2 \beta _j^2,\\ &{}Y_{12}^{j}=Y_{23}^{j}=0,\\ &{}2Y_{13}^{j}+Y_{22}^j=2\sigma _j-\left( 2 \left( \theta _j^0-\tau \widetilde{\theta }_j^0\right) \rho _j \omega _j+\left( b_0^j-\tau \widetilde{b}_0^j\right) \omega _j\right) \left( \beta _j+\gamma _j\right) \\ &{}~~~~~~~~~~~~~~~~~-2 \left( \theta _j^0-\tau \widetilde{\theta }_j^0\right) \omega _j^2 \beta _j\gamma _j,\\ &{}Y_{33}^{j}=\sigma _j-\left( 2\left( \theta _j^0-\tau \widetilde{\theta }_j^0\right) \rho _j \omega _j+\left( b_0^j-\tau \widetilde{b}_0^j\right) \omega _j\right) \gamma _j-\left( \theta _j^0-\tau \widetilde{\theta }_j^0\right) \omega _j^2 \gamma _j^2. \end{aligned} \end{aligned} \end{array}\right. \end{aligned}$$

Corollary 4.4

Consider the problem \(\mathrm {(FTP)}\) with \(\mathcal {U}^i,\) \(i=1,\dots ,l,\) given by (14) and its reformulation problem \(\mathrm {(FSDP)}\). Then,

$$\begin{aligned} \inf (\textrm{FTP})=\inf (\textrm{FSDP}). \end{aligned}$$

Moreover, \((x,\rho ,W)\) is an optimal solution of the problem \(\mathrm {(FTP)}\) if and only if there exist \(\tau \in \mathbb {R},\) \(\delta \in \mathbb {R}^{lm},\) \(u \in \mathbb {R}^{pl},\) \(X \in \mathbb {R}^{3\times 3lm},\) \(\sigma \in \mathbb {R}^m\) and \(Y \in \mathbb {R}^{3\times 3\,m}\) such that \((x,\rho ,W,\tau ,\delta ,u,X,\sigma ,Y)\) is an optimal solution of the problem \(\mathrm {(FSDP)}\).

Proof

From (15) and Theorem 4.1, it is easy to show the validity of the corollary.

Let us now consider the case when \(\mathcal {U}^i,\) \(i=1,\dots ,l,\) are boxes in (16). In this setting, the SDP reformulation of (FSDP) becomes

$$\begin{aligned} \left\{ \begin{array}{ll} \begin{aligned} &{}\inf \limits _{\begin{array}{c} x\in \mathbb {R}^n, \tau \in \mathbb {R}, \delta \in \mathbb {R}^{lm}, u\in \mathbb {R}^{pl}, \rho \in \mathbb {R}^m, \\ X\in \mathbb {R}^{3\times 3lm}, W\in \mathbb {R}^{m\times m}, \sigma \in \mathbb {R}^m, Y\in \mathbb {R}^{3\times 3m} \end{array}} ~\frac{x^{\top }Q_0x+q_0^{\top }x+\xi _0}{x^{\top }\widetilde{Q}_0x+\widetilde{q}_0^{\top }x+\widetilde{\xi }_0}+\tau \nonumber \\ &{}~~~~~~~~s.t.~~~~\sum _{j=1}^m\delta _i^j\le -\left( \sum _{s=1}^n a_{s}^ix_s^2+\sum _{s=1}^n \left( q^{s}_i+\sum _{k=1}^p u_k^i q^{is}_k\right) x_s+\xi _i+\sum _{k=1}^p u_k^i \xi _k^i\right. \nonumber \\ &{}~~~~~~~~~~~~~~~\left. +\sum _{j=1}^m \theta _j^i \rho _j^2+\sum _{j=1}^m b_i^j \rho _j+t_i\right) ,~i=1,\dots ,l, \nonumber \\ &{}~~~~~~~~~~~~~~~\lambda _i+u_k^i\ge 0,~\lambda _i-u_k^i\ge 0,~k=1,\dots ,p,~i=1,\dots ,l,\nonumber \\ &{}~~~~~~~~~~~~~~~X^{ij}:= \left( \begin{array}{lll} X_{11}^{ij} ~X_{12}^{ij} ~X_{13}^{ij}\\ X_{12}^{ij} ~X_{22}^{ij} ~X_{23}^{ij}\\ X_{13}^{ij} ~X_{23}^{ij} ~X_{33}^{ij} \end{array}\right) \succeq 0, ~i=1,\dots ,l, ~j=1,\dots ,m,\nonumber \\ &{}~~~~~~~~~~~~~~ \begin{aligned} &{}X_{11}^{ij}=\delta _i^j-\left( 2\rho _j \theta _j^i \omega _j+b_i^j \omega _j\right) \beta _j-\theta _j^i \omega _j^2 \beta _j^2,\\ &{}X_{12}^{ij}=X_{23}^{ij}=0,\\ &{}2X_{13}^{ij}+X_{22}^{ij}=2\delta _i^j-\left( 2\rho _j \theta _j^i \omega _j+b_i^j \omega _j\right) \left( \beta _j+\gamma _j\right) -2 \theta _j^i \omega _j^2 \beta _j\gamma _j,\\ &{}X_{33}^{ij}=\delta _i^j-\left( 2\rho _j \theta _j^i \omega _j+b_i^j \omega _j\right) \gamma _j-\theta _j^i \omega _j^2 \gamma _j^2, \end{aligned} \nonumber \\ &{}~~~~~~~~~~~~~~\sum _{j=1}^m \sigma _j\le \tau -\sum _{j=1}^m \left( \theta _j^0-\tau \widetilde{\theta }_j^0\right) \rho _j^2-\sum _{j=1}^m \left( b_0^j -\tau \widetilde{b}_0^j \right) \rho _j-\left( t_0-\tau \widetilde{t}_0\right) ,\nonumber \\ &{}~~~~~~~~~~~~~~~Y^{j}:= \left( \begin{array}{lll} Y_{11}^{j} ~Y_{12}^{j} ~Y_{13}^{j}\\ Y_{12}^{j} ~Y_{22}^{j} ~Y_{23}^{j}\\ Y_{13}^{j} ~Y_{23}^{j} ~Y_{33}^{j} \end{array}\right) \succeq 0, ~j=1,\dots ,m,\nonumber \\ &{}~~~~~~~~~~~~~~ \begin{aligned} &{}Y_{11}^{j}=\sigma _j-\left( 2 \left( \theta _j^0-\tau \widetilde{\theta }_j^0\right) \rho _j \omega _j+\left( b_0^j-\tau \widetilde{b}_0^j\right) \omega _j\right) \beta _j-\left( \theta _j^0-\tau \widetilde{\theta }_j^0\right) \omega _j^2 \beta _j^2,\\ &{}Y_{12}^{j}=Y_{23}^{j}=0,\\ &{}2Y_{13}^{j}+Y_{22}^j=2\sigma _j-\left( 2 \left( \theta _j^0-\tau \widetilde{\theta }_j^0\right) \rho _j \omega _j+\left( b_0^j-\tau \widetilde{b}_0^j\right) \omega _j\right) \left( \beta _j+\gamma _j\right) \\ &{}~~~~~~~~~~~~~~~~~-2 \left( \theta _j^0-\tau \widetilde{\theta }_j^0\right) \omega _j^2 \beta _j\gamma _j,\\ &{}Y_{33}^{j}=\sigma _j-\left( 2\left( \theta _j^0-\tau \widetilde{\theta }_j^0\right) \rho _j \omega _j+\left( b_0^j-\tau \widetilde{b}_0^j\right) \omega _j\right) \gamma _j-\left( \theta _j^0-\tau \widetilde{\theta }_j^0\right) \omega _j^2 \gamma _j^2. \end{aligned} \end{aligned} \end{array}\right. \end{aligned}$$

Corollary 4.5

Consider the problem \(\mathrm {(FTP)}\) with \(\mathcal {U}^i,\) \(i=1,\dots ,l,\) given by (16) and its reformulation problem \(\mathrm {(FSDP)}\). Then,

$$\begin{aligned} \inf (\textrm{FTP})=\inf (\textrm{FSDP}). \end{aligned}$$

Moreover, \((x,\rho ,W)\) is an optimal solution of the problem \(\mathrm {(FTP)}\) if and only if there exist \(\tau \in \mathbb {R},\) \(\delta \in \mathbb {R}^{lm},\) \(u \in \mathbb {R}^{pl},\) \(X \in \mathbb {R}^{3\times 3lm},\) \(\sigma \in \mathbb {R}^m\) and \(Y \in \mathbb {R}^{3\times 3\,m}\) such that \((x,\rho ,W,\tau ,\delta ,u,X,\sigma ,Y)\) is an optimal solution of the problem \(\mathrm {(FSDP)}\).

Proof

By (17) and Theorem 4.1, it is easy to obtain the desired result.

5 Conclusions

In this paper, under the framework of adjustable robust optimization approach, a class of adjustable robust quadratic optimization problems, where both objective and constraint functions involve uncertain data, is considered. We first obtain exact SDP reformulations for adjustable robust quadratic optimization problems with an affine decision rule on spectrahedral uncertain sets. For applications, we also establish exact SDP reformulations for fractionally adjustable robust quadratic optimization problems on spectrahedral uncertain sets.

In the future, further studies on SDP problems and optimality conditions for adjustable robust quadratic optimization problems are still needed. For example, similar to [11], can we obtain some SDP dual results for adjustable robust quadratic optimization problems with an affine decision rule. On the other hand, it is important to consider how the proposed approach can be extended to handle adjustable robust nonlinear optimization problems.