Hyper-minimisation Made Efficient

  • Authors:
  • Paweł Gawrychowski;Artur Jeż

  • Affiliations:
  • Institute of Computer Science, University of Wrocław, Poland;Institute of Computer Science, University of Wrocław, Poland

  • Venue:
  • MFCS '09 Proceedings of the 34th International Symposium on Mathematical Foundations of Computer Science 2009
  • Year:
  • 2009

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Abstract

We consider a problem of hyper-minimisation of an automaton [2,3]: given a DFA M we want to compute a smallest automaton N such that the language L(M) ΔL(N) is finite, where Δ denotes the symmetric difference. We improve the previously known $\mathcal O (|\Sigma|n^2)$ solution by giving an expected $\mathcal O (|\delta|\log n)$ time algorithm for this problem, where |驴| is the size of the (potentially partial) transition function. We also give a slightly slower deterministic $\mathcal O(|\delta|\log^2 n)$ version of the algorithm.Then we introduce a similar problem of k-minimisation: for an automaton M and number k we want to find a smallest automaton N such that L(M) ΔL(N) 驴 Σk , i.e. the languages they recognize differ only on words of length less than k. We characterise such minimal automata and give algorithm with a similar complexity for this problem.