Aperiodic monoid
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In mathematics, an aperiodic semigroup is a semigroup S such that for every x ∈ S, there exists a nonnegative integer n such that xn = xn + 1.
An aperiodic monoid is an aperiodic semigroup which is a monoid. This notion is in some sense orthogonal to that of group.
Recall that a subsemigroup G of a semigroup S is a subgroup of S (also called sometimes a group in S) if there exists an idempotent e such that G is a group with identity element e. A semigroup S is group-bound if some power of each element of S lies in some subgroup of S. Every finite semigroup is group-bound, but a group-bound semigroup might be infinite.
A finite semigroup is aperiodic if and only if it contains no nontrivial subgroups. In terms of Green's relations, a finite semigroup is aperiodic if and only if its H-relation is trivial. These two characterizations extend to group-bound semigroups.
A celebrated result of algebraic automata theory due to Marcel-Paul Schützenberger asserts that a language is star-free if and only if its syntactic monoid is finite and aperiodic.
A consequence of the Krohn-Rhodes theorem is that every finite aperiodic monoid divides a wreath product of copies of the three element monoid containing an identity element and two right zeros.
