The membership problem in aperiodic transformation monoids

  • Authors:
  • M. Beaudry;P. McKenzie;D. Thérien

  • Affiliations:
  • Univ. of Sherbrooke, Sherbrooke, P.Q., Canada;Univ. of Montreal, Montreal, P.Q., Canada;McGill Univ., Montreal, P.Q., Canada

  • Venue:
  • Journal of the ACM (JACM)
  • Year:
  • 1992

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Abstract

The problem of testing membership in aperiodic or “group-free” transformation monoids is the natural counterpart to the well-studied membership problem in permutation groups. The class A of all finite aperiodic monoids and the class G of all finite groups are two examples of varieties, the fundamental complexity units in terms of which finite monoids are classified. The collection of all varieties V forms an infinite lattice under the inclusion ordering, with the subfamily of varieties that are contained in A forming an infinite sublattice. For each V ⊆ A, the associated problem MEMB(V) of testing membership in transformation monoids that belong to V, is considered. Remarkably, the computational complexity of each such problem turns out to look familiar. Moreover, only five possibilities occur as V ranges over the whole aperiodic sublattice: With one family of NP-hard exceptions whose exact status is still unresolved, any such MEMB(V) is either PSPACE-complete, NP-complete, P-complete or in AC0. These results thus uncover yet another surprisingly tight link between the theory of monoids and computational complexity theory.