Weak inverse

In mathematics, the term weak inverse is used with several meanings.

Theory of semigroups

In the theory of semigroups, a weak inverse of an element x in a semigroup is an element y such that. If every element has a weak inverse, the semigroup is called an E-inversive or E-dense semigroup. An E-inversive semigroup may equivalently be defined by requiring that for every element, there exists such that and are idempotents.
An element x of S for which there is an element y of S such that is called regular. A regular semigroup is a semigroup in which every element is regular. This is a stronger notion than weak inverse. Every regular semigroup is E-inversive, but not vice versa.
If every element x in S has a unique inverse y in S in the sense that and then S is called an inverse semigroup.

Category theory

In category theory, a weak inverse of an object A in a monoidal category C with monoidal product ⊗ and unit object is an object B such that both and are isomorphic to the unit object of C. A monoidal category in which every morphism is invertible and every object has a weak inverse is called a 2-group.