# nLab coproduct

category theory

## Applications

#### Limits and colimits

limits and colimits

# Coproducts

## Idea

The notion of coproduct is a generalization to arbitrary categories of the notion of disjoint union in the category Set.

## Definition

For $C$ a category and $x, y \in Obj(C)$ two objects, their coproduct is an object $x \coprod y$ in $C$ equipped with two morphisms

$\array{ x &&&& y \\ & {}_{\mathllap{i_x}}\searrow && \swarrow_{\mathrlap{i_y}} \\ && x \coprod y }$

such that this is universal with this property, meaning such that for any other object with maps like this

$\array{ x &&&& y \\ & {}_{\mathllap{f}}\searrow && \swarrow_{\mathrlap{g}} \\ && Q }$

there exists a unique morphism $(f,g) : x \coprod y \to Q$ such that we have a commuting diagram

$\array{ x &\stackrel{i_x}{\to}& x \coprod y &\stackrel{i_y}{\leftarrow}& y \\ & {}_{\mathrlap{f}}\searrow & \downarrow^{\mathrlap{(f,g)}} & \swarrow_{\mathrlap{g}} \\ && Q } \,.$

This morphism $(f,g)$ is called the copairing of $f$ and $g$.

Notation. The coproduct is also denoted $a+b$ or $a\amalg b$, especially when it is disjoint (or $a \sqcup b$ if your fonts don't include ‘$\amalg$’). The copairing is also denoted $[f,g]$ or (when possible) given vertically: $\left\{{f \atop g}\right\}$.

A coproduct is thus the colimit over the diagram that consists of just two objects.

More generally, for $S$ any set and $F : S \to C$ a collection of objects in $C$ indexed by $S$, their coproduct is an object

$\coprod_{s \in S} F(s)$

equipped with maps

$F(s) \to \coprod_{s \in S} F(s)$

such that this is universal among all objects with maps from the $F(s)$.

## Examples

• In Set, the coproduct of a family of sets $(C_i)_{i\in I}$ is the disjoint union $\coprod_{i\in I} C_i$ of sets.

This makes the coproduct a categorification of the operation of addition of natural numbers and more generally of cardinal numbers: for $S,T \in FinSet$ two finite sets and $|-| : FinSet \to \mathbb{N}$ the cardinality operation, we have

$|S \coprod T| = |S| + |T| \,.$
• In Top, the coproduct of a family of spaces $(C_i)_{i\in I}$ is the space whose set of points is $\coprod_{i\in I} C_i$ and whose open subspaces are of the form $\coprod_{i\in I} U_i$ (the internal disjoint union) where each $U_i$ is an open subspace of $C_i$. This is typical of topological concrete categories.

• In Grp, the coproduct is the free product, whose underlying set is not a disjoint union. This is typical of algebraic categories.

• In Ab, in Vect, the coproduct is the subobject of the product consisting of those tuples of elements for which only finitely many are not 0.

• In Cat, the coproduct of a family of categories $(C_i)_{i\in I}$ is the category with

$Obj(\coprod_{i\in I} C_i) = \coprod_{i\in I} Obj(C_i)$

and

Hom_{\coprod_{i\in I} C_i}(x,y) = \left\{ \begin{aligned} Hom_{C_i}(x,y) & if x,y \in C_i \\ \emptyset & otherwise \end{aligned} \right.
• In Grpd, the coproduct follows Cat rather than Grp. This is typical of oidifications: the coproduct becomes a disjoint union again.

## Properties

• A coproduct in $C$ is the same as a product in the opposite category $C^{op}$.

• When they exist, coproducts are unique up to unique canonical isomorphism, so we often say “the coproduct.”

• A coproduct indexed by the empty set is an initial object in $C$.

Revised on February 14, 2013 23:47:41 by Anonymous Coward (173.64.113.16)