Contents

category theory

# Contents

## Definition

An inverse of a morphism $f : X \to Y$ in a category or unital magmoid (or an element of a monoid or unital magma) is another morphism $f^{-1} : Y \to X$ which is both a left-inverse (a retraction) as well as a right-inverse (a section) of $f$, in that

$f \circ f^{-1} : Y \to Y$

equals the identity morphism on $Y$ and

$f^{-1} \circ f : X \to X$

equals the identity morphism on $X$.

## Remarks

• A morphism which has an inverse is called an isomorphism.

• The inverse $f^{-1}$ is unique if it exists.

• The inverse of an inverse morphism is the original morphism, $(f^{-1})^{-1} = f$.

• An identity morphism, $i$, is a morphism which is its own inverse: $i^{-1} = i$.

• A category in which all morphisms have inverses is called a groupoid.

• An amusing exercise is to show that if $f,g,h$ are morphisms such that $f\circ g,\; g\circ h$ are defined and are isomorphisms, then $f,g,h$ are all isomorphisms.

• In a balanced category, such as a topos or more particularly Set, every morphism that is both monic and epic is an isomorphism and thus has an inverse. A partial order is an unbalanced category where every morphism is both monic and epic. Only its identity morphisms have inverses.

## In non-unital contexts

In a magmoid or semicategory (or an element of a semigroup or magma), a morphism $f:a \to b$ has a unique retraction $f^{-1}:b \to a$ if

• for every morphism $g:b \to c$, $g \circ (f^{-1} \circ f) = g$,

• for every morphism $g:c \to a$, $(f^{-1} \circ f) \circ g = g$,

and a morphism $f:a \to b$ has a unique section $f^{-1}:b \to a$ if

• for every morphism $g:a \to c$, $g \circ (f \circ f^{-1}) = g$,

• for every morphism $g:a \to c$, $(f \circ f^{-1})\circ g = g$,

A morphism $f:a \to b$ has a unique inverse if it has a retraction that is also a section.

Last revised on May 25, 2021 at 09:36:53. See the history of this page for a list of all contributions to it.