Presheaf (category theory)
# Presheaf (category theory)

In category theory, a branch of mathematics, a presheaf on a category $C$ is a functor $F\colon C^{\mathrm {op} }\to \mathbf {Set}$. If $C$ is the poset of open sets in a topological space, interpreted as a category, then one recovers the usual notion of presheaf on a topological space.

A morphism of presheaves is defined to be a natural transformation of functors. This makes the collection of all presheaves on $C$ into a category, and is an example of a functor category. It is often written as ${\widehat {C}}=\mathbf {Set} ^{C^{\mathrm {op} }}$. A functor into ${\widehat {C}}$ is sometimes called a profunctor.

A presheaf that is naturally isomorphic to the contravariant hom-functor Hom(–, A) for some object A of C is called a representable presheaf.

Some authors refer to a functor $F\colon C^{\mathrm {op} }\to \mathbf {V}$ as a $\mathbf {V}$-valued presheaf.

## Examples

* A simplicial set is a Set-valued presheaf on the simplex category $C=\Delta$.

## Properties

* When $C$ is a small category, the functor category ${\widehat {C}}=\mathbf {Set} ^{C^{\mathrm {op} }}$ is cartesian closed.
* The poset of subobjects of $P$ form a Heyting algebra, whenever $P$ is an object of ${\widehat {C}}=\mathbf {Set} ^{C^{\mathrm {op} }}$ for small $C$.
* For any morphism $f:X\to Y$ of ${\widehat {C}}$, the pullback functor of subobjects $f^{*}:\mathrm {Sub} _{\widehat {C}}(Y)\to \mathrm {Sub} _{\widehat {C}}(X)$ has a right adjoint, denoted $\forall _{f}$, and a left adjoint, $\exists _{f}$. These are the universal and existential quantifiers.
* A locally small category $C$ embeds fully and faithfully into the category ${\widehat {C}}$ of set-valued presheaves via the Yoneda embedding which to every object $A$ of $C$ associates the hom functor $C(-,A)$.
* The category ${\widehat {C}}$ admits small limits and small colimits. See limit and colimit of presheaves for further discussion.
* The density theorem states that every presheaf is a colimit of representable presheaves; in fact, ${\widehat {C}}$ is the colimit completion of $C$ (see #Universal property below.)

## Universal property

The construction $C\mapsto {\widehat {C}}=\mathbf {Fct} (C^{\text{op}},\mathbf {Set} )$ is called the colimit completion of C because of the following universal property:

Proposition — Let C, D be categories and assume D admits small colimits. Then each functor $\eta :C\to D$ factorizes as

    $C{\overset {y}{\longrightarrow }}{\widehat {C}}{\overset {\widetilde {\eta }}{\longrightarrow }}D$

where y is the Yoneda embedding and ${\widetilde {\eta }}:{\widehat {C}}\to D$ is a, unique up to isomorphism, colimit-preserving functor called the Yoneda extension of $\eta$.

Proof: Given a presheaf F, by the density theorem, we can write $F=\varinjlim yU_{i}$ where $U_{i}$ are objects in C. Then let ${\widetilde {\eta }}F=\varinjlim \eta U_{i},$ which exists by assumption. Since $\varinjlim -$ is functorial, this determines the functor ${\widetilde {\eta }}:{\widehat {C}}\to D$. Succinctly, ${\widetilde {\eta }}$ is the left Kan extension of $\eta$ along y; hence, the name "Yoneda extension". To see ${\widetilde {\eta }}$ commutes with small colimits, we show ${\widetilde {\eta }}$ is a left-adjoint (to some functor). Define ${\mathcal {H}}om(\eta ,-):D\to {\widehat {C}}$ to be the functor given by: for each object M in D and each object U in C,

    ${\mathcal {H}}om(\eta ,M)(U)=\operatorname {Hom} _{D}(\eta U,M).$

Then, for each object M in D, since ${\mathcal {H}}om(\eta ,M)(U_{i})=\operatorname {Hom} (yU_{i},{\mathcal {H}}om(\eta ,M))$ by the Yoneda lemma, we have:

    ${\begin{aligned}\operatorname {Hom} _{D}({\widetilde {\eta }}F,M)&=\operatorname {Hom} _{D}(\varinjlim \eta U_{i},M)=\varprojlim \operatorname {Hom} _{D}(\eta U_{i},M)=\varprojlim {\mathcal {H}}om(\eta ,M)(U_{i})\\&=\operatorname {Hom} _{\widehat {C}}(F,{\mathcal {H}}om(\eta ,M)),\end{aligned}}$

which is to say ${\widetilde {\eta }}$ is a left-adjoint to ${\mathcal {H}}om(\eta ,-)$. $\square$

The proposition yields several corollaries. For example, the proposition implies that the construction $C\mapsto {\widehat {C}}$ is functorial: i.e., each functor $C\to D$ determines the functor ${\widehat {C}}\to {\widehat {D}}$.

## Variants

A presheaf of spaces on an ∞-category C is a contravariant functor from C to the ∞-category of spaces (for example, the nerve of the category of CW-complexes.) It is an ∞-category version of a presheaf of sets, as a "set" is replaced by a "space". The notion is used, among other things, in the ∞-category formulation of Yoneda's lemma that says: $C\to PShv(C)$ is fully faithful (here C can be just a simplicial set.)

