category theory

# Contents

## Idea

The concept of isomorphism generalizes the concept of bijection from the category Set of sets to general categories.

An isomorphism is an invertible morphism, hence a morphism with an inverse morphism.

Two objects of a category are said to be isomorphic if there exists an isomorphism between them. This means that they “are the same for all practical purposes” as long as one does not violate the principle of equivalence.

But beware that two objects may be isomorphic by more than one isomorphism. In particular a single object may be isomorphic to itself by nontrivial isomorphisms other than the identity morphism. Frequently the particular choice of isomorphism matters.

Every isomrophism is in particular an epimorphism and a monomorphism, but the converse need not hold.

Common jargon includes “is a mono” or “is monic” for “is a monomorphism”, and “is an epi” or “is epic” for “is an epimorphism”, and “is an iso” for “is an isomorphism”.

## Definitions

An isomorphism, or iso for short, is an invertible morphism, i.e. a morphism with a 2-sided inverse.

A morphism could be called isic (following the more common ‘monic’ and ‘epic’) if it is an isomorphism, but it's more common to simply call it invertible. Two objects $x$ and $y$ are isomorphic if there exists an isomorphism from $x$ to $y$ (or equivalently, from $y$ to $x$). An automorphism is an isomorphism from one object to itself.

## Properties

It is immediate that isomorphisms satisfy the two-out-of-three property. But they also satisfy two-out-of-six property.

Note that the inverse of an isomorphism is an isomorphism, as is any identity morphism or composite of isomorphisms. Thus, being isomorphic is an equivalence relation on objects. The equivalence classes form the fundamental 0-groupoid of the category in question.

Every isomorphism is both a split monomorphism (and thus about any other kind of monomorphism) and a split epimorphism (and thus about any other kind of epimorphism). In a balanced category, every morphism that is both monic and epic (called a bimorphism) is invertible, but this does not hold in general. However, any monic regular epimorphism (or dually, any epic regular monomorphism) must be an isomorphism.

A groupoid is precisely a category in which every morphism is an isomorphism. More generally, the core of any category $C$ is the subcategory consisting of all objects but only the isomorphisms; it is a kind of underlying groupoid of $C$. In a similar way, the automorphisms of any given object $x$ form a group, the automorphism group of $x$.

In higher categories, isomorphisms generalise to equivalences, which we expect to have only weak inverses.

## Examples

Revised on May 8, 2017 07:52:39 by Urs Schreiber (176.74.57.162)