Reversal hierarchies for small 2DFAs

  • Authors:
  • Christos A. Kapoutsis;Giovanni Pighizzini

  • Affiliations:
  • LIAFA, Université Paris VII, France;DI, Università degli Studi di Milano, Italia

  • Venue:
  • MFCS'12 Proceedings of the 37th international conference on Mathematical Foundations of Computer Science
  • Year:
  • 2012

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Abstract

A two-way deterministic finite automaton with r(n) reversals performs ≤r (n) input head reversals on every n-long input. Let 2D[r(n)] be all families of problems solvable by such automata of size polynomial in the index of the family. Then the reversal hierarchy 2D[0] ⊆ 2D[1] ⊆ 2D[2] ⊆⋯ is strict, but 2D[O(1)] = 2D[o(n)]. Moreover, the inner-reversal hierarchy 2D(0) ⊆ 2D(1) ⊆ 2D(2) ⊆⋯ , where now the bound is only for reversals strictly between the input end-markers, is also strict.