Abstract
A word w over an alphabet Σ is n-synchronizing if it resets every (n + 1)-state synchronizing automaton over this alphabet. For a fixed n and Σ, n-synchronizing words can be recognized in polynomial time, yet no practical algorithm is known. In this paper we show that one cannot expect to find such an algorithm. We prove that the problem of recognizing 2-synchronizing words, where the input consists of a word and an alphabet, is co-NP-complete. We also show that the length of a 2-synchronizing word is at least 2|Σ|2, which improves the lower bound known so far.
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Gawrychowski, P., Kisielewicz, A. (2008). 2-Synchronizing Words. In: Martín-Vide, C., Otto, F., Fernau, H. (eds) Language and Automata Theory and Applications. LATA 2008. Lecture Notes in Computer Science, vol 5196. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-540-88282-4_21
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DOI: https://doi.org/10.1007/978-3-540-88282-4_21
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