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The paper completely characterizes the primality of acyclic DFAs, where a DFA 𝒜 is prime if there do not exist DFAs 𝒜_1,… ,𝒜_t with ℒ(𝒜) = ⋂_{i=1}^t ℒ(𝒜_i) such that each 𝒜_i has strictly less states than the minimal DFA recognizing the same language as 𝒜. A regular language is prime if its minimal DFA is prime. Thus, this result also characterizes the primality of finite languages.
Further, the NL-completeness of the corresponding decision problem Prime-DFA_fin is proven. The paper also characterizes the primality of acyclic DFAs under two different notions of compositionality, union and union-intersection compositionality.
Additionally, the paper introduces the notion of S-primality, where a DFA 𝒜 is S-prime if there do not exist DFAs 𝒜₁,… ,𝒜_t with ℒ(𝒜) = ⋂_{i=1}^t ℒ(𝒜_i) such that each 𝒜_i has strictly less states than 𝒜 itself. It is proven that the problem of deciding S-primality for a given DFA is NL-hard. To do so, the NL-completeness of 2Minimal-DFA, the basic problem of deciding minimality for a DFA with at most two letters, is proven.
@InProceedings{spenner:LIPIcs.MFCS.2023.83,
author = {Spenner, Daniel Alexander},
title = {{Decomposing Finite Languages}},
booktitle = {48th International Symposium on Mathematical Foundations of Computer Science (MFCS 2023)},
pages = {83:1--83:14},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-292-1},
ISSN = {1868-8969},
year = {2023},
volume = {272},
editor = {Leroux, J\'{e}r\^{o}me and Lombardy, Sylvain and Peleg, David},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.MFCS.2023.83},
URN = {urn:nbn:de:0030-drops-186173},
doi = {10.4230/LIPIcs.MFCS.2023.83},
annote = {Keywords: Deterministic finite automaton (DFA), Regular languages, Finite languages, Decomposition, Primality, Minimality}
}