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arXiv · 1704.07860

Universality of Confluent, Self-Loop Deterministic Partially Ordered NFAs is Hard

Tomáš Masopust·Markus Krötzsch
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Abstract

An automaton is partially ordered if the only cycles in its transition diagram are self-loops. The expressivity of partially ordered NFAs (poNFAs) can be characterized by the Straubing-Th\'erien hierarchy. Level 3/2 is recognized by poNFAs, level 1 by confluent, self-loop deterministic poNFAs as well as by confluent poDFAs, and level 1/2 by saturated poNFAs. We study the universality problem for confluent, self-loop deterministic poNFAs. It asks whether an automaton accepts all words over its alphabet. Universality for both NFAs and poNFAs is a PSpace-complete problem. For confluent, self-loop deterministic poNFAs, the complexity drops to coNP-complete if the alphabet is fixed but is open if the alphabet may grow. We solve this problem by showing that it is PSpace-complete if the alphabet may grow polynomially. Consequently, our result provides a lower-bound complexity for some other problems, including inclusion, equivalence, and $k$-piecewise testability. Since universality for saturated poNFAs is easy, confluent, self-loop deterministic poNFAs are the simplest and natural kind of NFAs characterizing a well-known class of languages, for which deciding universality is as difficult as for general NFAs.

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Tomáš Masopust, Markus Krötzsch. 2017-04-25. Universality of Confluent, Self-Loop Deterministic Partially Ordered NFAs is Hard. https://arxiv.org/abs/1704.07860

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