A function $f$ from $A$ to $B$ is **injective** if $x = y$ whenever $f(x) = f(y)$. Equivalently, a function is injective if all its fibers are subsingletons: for all elements $b \in B$ and for all elements $x \in A$ and $y \in A$, if $f(x) = b$ and $f(y) = b$, then $x = y$. An injective function is also called **one-to-one** or an **injection**; it is the same as a monomorphism in the category of sets.

A *bijection* is a function that is both injective and surjective.

In constructive mathematics, a strongly extensional function between sets equipped with tight apartness relations is called **strongly injective** if $f(x) \ne f(y)$ whenever $x \ne y$ (which implies that the function is injective). This is the same as a regular monomorphism in the category of such sets and strongly extensional functions (while any merely injective function, if strongly extensional, is still a monomorphism). Some authors use ‘one-to-one’ for an injective function as defined above and reserve ‘injective’ for the stronger notion.

Since an element $a$ in a set $A$ in the category of sets is just a global element $a:1\rightarrow A$, one could define injections in any category $\mathcal{C}$ with a terminal object $1$:

A morphism $f:A\rightarrow B$ in $\mathcal{C}$ is an **injection** or a **one-to-one morphism** if, given any two global elements $x, y:1\rightarrow A$, $x = y$ if $f \circ x = f \circ y$.

The term **injective morphism** is already used in category theory in a different context to mean a morphism with a right lifting property.

In a category $\mathcal{C}$ with a terminal object $1$, every monomorphism is an injection.

This follows from the definition of a monomorphism.

In a category $\mathcal{C}$ with a terminal object $1$, every global element $e:1\rightarrow A$ is an injection.

By definition of terminal object $1$, the unique global element $i:1\rightarrow 1$ is the identity morphism of the terminal object. Thus for every global element $e:1\rightarrow A$, for any two global elements $x, y:1\rightarrow 1$, $x = y$ is always true, making $e:1\rightarrow A$ an injection.

If the category has a strict initial object $\emptyset$, then every morphism $f:\emptyset\rightarrow B$ is vacuously an injection, since there are no global elements $x:1\rightarrow\emptyset$.

Last revised on August 2, 2023 at 17:28:05. See the history of this page for a list of all contributions to it.