nLab surjection

Definition

A function $f$ (of sets) from $A$ to $B$ is surjective if, given any element $y$ of $B$, $y = f(x)$ for some $x$. A surjective function is also called onto or a surjection.

Surjections are the same as epimorphisms in the category of sets and effective epimorphisms in the (infinity,1)-category of sets.

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

The surjection preorder

In classical set theory, one writes $|B| \leq^* |A|$ to mean that either there is a surjection $A \to B$ or $B=\empty$. The relation $\leq^*$ is a preorder on the class of all sets, and its restriction to inhabited sets is the preorder reflection of the category $Surj_{inh}$ of inhabited sets and surjections.

To make this definition less piecemeal and more constructive, one can define $|B| \leq^* |A|$ to mean $B$ is a subquotient of $A$, in other words one has a surjection $B' \to B$ and an injection $B' \to A$. For $B$ inhabited and a subquotient of $A$, excluded middle implies there is a surjection from $A$ to $B$, so in classical mathematics this coincides with the piecemeal definition.

Contrast with the notation $|B| \leq |A|$ if there is an injection $B\to A$. Since subobjects are subquotients (e.g. we can take $B'=B$ above), $|B| \leq |A|$ implies $|B| \leq^* |A|$. (If we wanted to prove this using the piecemeal definition, we would require excluded middle.)

Axioms of choice

The axiom of choice states precisely that every surjection in the category of sets has a section. Thus in this setting one has: $|B| \leq^* |A|$ implies $|B| \leq |A|$, and so $|B| \leq^* |A|$ iff $|B| \leq |A|$ assuming AC. Some authors who doubt the axiom of choice use the term ‘onto’ for a surjection as defined above and reserve ‘surjective’ for the stronger notion of a function with a section (a split epimorphism).

The axiom WISC has an equivalent statement (that works in any Boolean topos) due to François Dorais phrased almost entirely in terms of surjections (or epimorphisms):

For every set $X$ there is a set $Y$ such that for every surjection $q\colon Z \to X$ there is a function $s\colon Y \to Z$ such that $q\circ s\colon Y\to X$ is a surjection.

One can view this as really a statement about the Grothendieck fibration over Set with fibre over $X$ the full subcategory of $Set/X$ on the surjections: every fibre has a weakly initial object.

In other categories

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 surjections in any category $\mathcal{C}$ with a terminal object $1$:

Definition

A morphism $f:A\rightarrow B$ in a category $\mathcal{C}$ with a terminal object $1$ is called a surjection. a surjective morphism, or an onto morphism if, given any global element $y:1\rightarrow B$, there exists a global element $x:1\rightarrow A$ such that $y = f \circ x$.

Remark

Some authors regard surjection as a synonym of split epimorphism, and only use ‘onto’ for the definition above.

Proposition

In a category $\mathcal{C}$ with a terminal object $1$, the unique morphism $!:A\rightarrow 1$ is a surjection for every object $A$.

Proof

By definition of a terminal object, for every object $A$ there exists a unique morphism $!:A\rightarrow 1$, and the identity morphism is the unique global element $1_{1}:1\rightarrow 1$. The composite of a global element $x:1\rightarrow A$ with the function $!:A\rightarrow 1$ results in a function $! \circ x:1\rightarrow 1$, which by definition of a terminal object is the same as $1_{1}:1\rightarrow 1$. Since $! \circ x = 1_{1}$, for every object $A$, $!:A\rightarrow 1$ is a surjection.

Proposition

In a category of pointed objects $\mathcal{C}$, every morphism $f:A\rightarrow B$ is a surjection for every object $A$.

Proof

A pointed object in a category with terminal objects is a object $A$ with a global element $a:1\rightarrow A$ to the object, and morphisms in the category preserve the global element, which means there is only a unique global element for each object $A$. Therefore, for every morphism $f:A\rightarrow B$ and global element $y:1\rightarrow B$, there exists a global element $x:1\rightarrow A$ such that $y = f \circ x$.

Just because a morphism is a surjection in a concrete category does not mean that the morphism in the underlying category of sets is a surjection; equivalently, the forgetful functor from any category $\mathcal{C}$ to Set does not preserve surjections.

Well-pointedness

The fact that surjections are epimorphisms in Set is a result of the fact that Set is well-pointed. This could be generalised to any category with a terminal separator $1$.

Proposition

In a category $\mathcal{C}$ with a terminal object $1$ such that $1$ is a separator, every surjection is an epimorphism.

Proof

For any surjection $f:A\rightarrow B$, suppose there are parallel morphisms $g, h:B\rightarrow C$ such that there $g \circ f = h \circ f$ (a fork). Then for every global element $y:1\rightarrow B$ there exists a global element $x:1\rightarrow A$ such that $y = f \circ x$, and thus $g \circ y = g \circ f \circ x$ and $h \circ y = h \circ f \circ x$. But since $g \circ f = h \circ f$, $g \circ f \circ x = h \circ f \circ x$, which implies that $g \circ y = h \circ y$. Since $1$ is a separator, then for every global element $y:1\rightarrow B$, if $g \circ y = h \circ y$, then $g = h$. Therefore, every surjection is an epimorphism.

Axiom of choice

The axiom of choice for surjections in Set is the following statement:

• Every surjection is a split epimorphism.

This axiom could be defined in every category with a terminal object, and could be contrasted with the axiom of choice for epimorphisms, as not every epimorphism is an surjection, nor is every surjection an epimorphism.

Proposition

In a category $\mathcal{C}$ with a terminal object $1$ and binary equalisers such that every surjection is a split epimorphism, the terminal object $1$ is a separator.

Proof

Suppose there are parallel morphisms $g, h:B\rightarrow C$ such that for every global element $y:1\rightarrow B$, $g \circ y = h \circ y$. One could construct an equaliser $f:eq(g,h)\rightarrow B$, which implies that $g \circ f = h \circ f$ and that for any global element $x:1\rightarrow eq(g,h)$, $g \circ f \circ x = h \circ f \circ x$. This implies that for every global element $y:1\rightarrow B$, there exists a global element $x:1\rightarrow eq(g,h)$ where $y = f \circ x$, and the equaliser $f:eq(g,h)\rightarrow B$ is a surjection. Since every surjection is a split epimorphism, $f$ has a section $i:B\rightarrow eq(g,h)$ such that $f \circ i = 1_{B}$, the identity morphism on $B$, $g \circ f \circ i = h \circ f \circ i$, $g \circ 1_{B} = h \circ 1_{B}$, and $g = f$. Because for every global element $y:1\rightarrow B$, $g \circ y = h \circ y$ implies $g = h$, the terminal object $1$ is a separator.

Related concepts

• injection

• epimorphism

• subquotient

• monotone function

• equivariant function