Abstract
Černý’s conjecture asserts the existence of a synchronizing word of length at most (n − 1)2 for any synchronized n-state deterministic automaton. We prove a quadratic upper bound on the length of a synchronizing word for any synchronized n-state deterministic automaton satisfying the following additional property: there is a letter a such that for any pair of states p,q, one has p ·a r = q ·a s for some integers r,s (for a state p and a word w, we denote by p ·w the state reached from p by the path labeled w). As a consequence, we show that for any finite synchronized prefix code with an n-state decoder, there is a synchronizing word of length O(n 2). This applies in particular to Huffman codes.
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Béal, MP., Perrin, D. (2009). A Quadratic Upper Bound on the Size of a Synchronizing Word in One-Cluster Automata. In: Diekert, V., Nowotka, D. (eds) Developments in Language Theory. DLT 2009. Lecture Notes in Computer Science, vol 5583. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-02737-6_6
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