Undecidability of Algebras of Binary Relations

Abstract

Let S be a signature of operations and relations definable in relation algebra (e.g. converse, composition, containment, union, identity, etc.), let R(S) be the class of all S-structures isomorphic to concrete algebras of binary relations with concrete interpretations for symbols in S, and let F(S) be the class of S-structures isomorphic to concrete algebras of binary relations over a finite base. To prove that membership of R(S) or F(S) for finite S-structures is undecidable, we reduce from a known undecidable problem—here we use the tiling problem, the partial group embedding problem and the partial group finite embedding problem to prove undecidability of finite membership of R(S) or F(S) for various signatures S. It follows that the equational theory of R(S) is undecidable whenever S includes the boolean operators and composition. We give an exposition of the reduction from the tiling problem and the reduction from the group embedding problem, and summarize what we know about the undecidability of finite membership of R(S) and of F(S) for different signatures S.