The **diagonal** of an object $X$ in a category with Cartesian product is the canonical morphism

$\Delta
\;\colon\;
X
\stackrel{(Id,Id)}{\longrightarrow}
X \times X$

which is induced, via the universal property of the Cartesian product, by the span whose two legs each are both the identity morphism on $X$:

$\array{
&& X
\\
&
{}^{\mathllap{Id}}\swarrow
&
\downarrow
{}^{\mathrlap{\Delta}}
&
\searrow^{\mathrlap{Id}}
\\
X
&\underset{pr_1}{\longleftarrow}&
X \times X
&\underset{pr_2}{\longrightarrow}&
X
}$

The dual concept is *codiagonal* .

In the absence of Cartesian products, or when intentionally disregarding them, diagonal morphisms may still be considered in a generalized sense in monoidal categories with diagonals.

Recall that the diagonal of a set is a subset of its cartesian square $X^2$. If $X$ is now an object in some cartesian monoidal category $C$, then the diagonal of $X$ is now a subobject of its categorial square $X^2$. (Actually, $C$ need not be cartesian monoidal, as long as the product $X \times X$ exists.)

Specifically, the **diagonal morphism** of $X$ is the morphism $\Delta_X: X \to X^2$ to the cartesian product of $X$ with itself given (using the universal property of the cartesian product) by the identity morphism from $X$ to itself, taken twice. That is, $\Delta_X$ is the universal solution to

$\array {
& & X \\
& \swarrow & \downarrow _ { \Delta _ X } & \searrow \\
X & & X ^ 2 & & X \\
\downarrow _ { \id _ X } & \swarrow & & \searrow & \downarrow _ { \id _ X } \\
X & & & & X
}$

If $C$ is $Set$ (the category of sets), then this diagonal morphism is precisely the diagonal function of $X$.

The diagonal morphism is always a regular monomorphism, since it is the equaliser of the two projection maps $X^2 \to X$. (In fact, it is a split monomorphism, since it is also a section of either projection map.) Thus, it makes $X$ into a regular subobject of $X^2$, the **diagonal subobject**. When $C$ is the $Set$, this recovers the original notion of the diagonal subset of $X^2$.

In any category with binary pullbacks, the kernel pair of the identity morphism $id$ on an object $X$ is the diagonal morphism $(id,id)$ of $X$, and has a coequalizer isomorphic to $X$ itself.

In the category Set the diagonal $\Delta_X$ is the function $a \mapsto (a,a)$ for all $a \in X$. See diagonal subset.

In the category Top of topological spaces, an object $X$ is a Hausdorff space if and only if its diagonal subobject is a closed subspace of $X^2$; this fact can be generalised to other notions of space.

In Cat the diagonal morphisms are diagonal functors.

Last revised on July 10, 2021 at 15:36:10. See the history of this page for a list of all contributions to it.