LIPIcs.MFCS.2023.83.pdf
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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.
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