Brzozowski, Janusz; Jirásková, Galina; Zou, Chenglong Quotient complexity of closed languages. (English) Zbl 1380.68249 Theory Comput. Syst. 54, No. 2, 277-292 (2014). Summary: A language \(L\) is prefix-closed if, whenever a word \(w\) is in \(L\), then every prefix of \(w\) is also in \(L\). We define suffix-, factor-, and subword-closed languages in an analogous way, where by factor we mean contiguous subsequence, and by subword we mean scattered subsequence. We study the state complexity (which we prefer to call quotient complexity) of operations on prefix-, suffix-, factor-, and subword-closed languages. We find tight upper bounds on the complexity of the subword-closure of arbitrary languages, and on the complexity of boolean operations, concatenation, star, and reversal in each of the four classes of closed languages. We show that repeated applications of positive closure and complement to a closed language result in at most four distinct languages, while Kleene closure and complement give at most eight. Cited in 13 Documents MSC: 68Q45 Formal languages and automata Keywords:closed language; finite automaton; quotient complexity; regular language; state complexity PDF BibTeX XML Cite \textit{J. Brzozowski} et al., Theory Comput. Syst. 54, No. 2, 277--292 (2014; Zbl 1380.68249) Full Text: DOI arXiv OpenURL References: [1] Ang, T.; Brzozowski, J., Languages convex with respect to binary relations, and their closure properties, Acta Cybern., 19, 445-464, (2009) · Zbl 1199.68168 [2] Avgustinovich, S.V.; Frid, A.E., A unique decomposition theorem for factorial languages, Int. J. Algebra Comput., 15, 149-160, (2005) · Zbl 1102.68052 [3] Bassino, F.; Giambruno, L.; Nicaud, C.; López-Ortiz, A. 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