In search of most complex regular languages

  • Authors:
  • Janusz Brzozowski

  • Affiliations:
  • David R. Cheriton School of Computer Science, University of Waterloo, Waterloo, ON, Canada

  • Venue:
  • CIAA'12 Proceedings of the 17th international conference on Implementation and Application of Automata
  • Year:
  • 2012

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Abstract

Regular languages that are most complex under common complexity measures are studied. In particular, certain ternary languages Un(a,b,c), n ≥ 3, over the alphabet {a,b,c} are examined. It is proved that the state complexity bounds that hold for arbitrary regular languages are also met by the languages Un(a,b,c) for union, intersection, difference, symmetric difference, product (concatenation) and star. Maximal bounds are also met by Un(a,b,c) for the number of atoms, the quotient complexity of atoms, the size of the syntactic semigroup, reversal, and 22 combined operations, 5 of which require slightly modified versions. The language Un(a,b,c,d) is an extension of Un(a,b,c), obtained by adding an identity input to the minimal DFA of Un(a,b,c). The witness Un(a,b,c,d) and its modified versions work for 14 more combined operations. Thus Un(a,b,c) and Un(a,b,c,d) appear to be universal witnesses for alphabets of size 3 and 4, respectively.