
Epigroup

In abstract algebra, an epigroup is a semigroup in which every element has a power that belongs to a subgroup. Formally, for all "x" in a semigroup "S", there exists a positive integer "n" and a subgroup "G" of "S" such that "x""n" belongs to "G".
Epigroups are known by wide variety of other names, including quasi-periodic semigroup, group-bound semigroup, completely π-regular semigroup, strongly π-regular semigroup (sπr), or just π-regular semigroup (although the latter is ambiguous).
More generally, in an arbitrary semigroup an element is called "group-bound" if it has a power that belongs to a subgroup.
Epigroups have applications to ring theory. Many of their properties are studied in this context.
Epigroups were first studied by Douglas Munn in 1961, who called them "pseudoinvertible".
Structure.
By analogy with periodic semigroups, an epigroup "S" is partitioned in classes given by its idempotents, which act as identities for each subgroup. For each idempotent "e" of "S", the set: formula_1 is called a "unipotency class" (whereas for periodic semigroups the usual name is torsion class.)
Subsemigroups of an epigroup need not be epigroups, but if they are, then they are called subepigroups. If an epigroup "S" has a partition in unipotent subepigroups (i.e. each containing a single idempotent), then this partition is unique, and its components are precisely the unipotency classes defined above; such an epigroup is called "unipotently partionable". However, not every epigroup has this property. A simple counterexample is the Brandt semigroup with five elements "B2" because the unipotency class of its zero element is not a subsemigroup. "B2" is actually the quintessential epigroup that is not unipotently partionable. An epigroup is unipotently partionable if and only if it contains no subsemigroup that is an ideal extension of a unipotent epigroup by "B2".
See also.
Special classes of semigroups