# Presheaf (category theory)

In category theory, a branch of mathematics, a presheaf on a category ${\displaystyle C}$ is a functor ${\displaystyle F\colon C^{\mathrm {op} }\to \mathbf {Set} }$. If ${\displaystyle C}$ 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 ${\displaystyle C}$ into a category, and is an example of a functor category. It is often written as ${\displaystyle {\widehat {C}}=\mathbf {Set} ^{C^{\mathrm {op} }}}$. A functor into ${\displaystyle {\widehat {C}}}$ 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 ${\displaystyle F\colon C^{\mathrm {op} }\to \mathbf {V} }$ as a ${\displaystyle \mathbf {V} }$-valued presheaf.[1]

## Properties

• When ${\displaystyle C}$  is a small category, the functor category ${\displaystyle {\widehat {C}}=\mathbf {Set} ^{C^{\mathrm {op} }}}$  is cartesian closed.
• The partially ordered set of subobjects of ${\displaystyle P}$  form a Heyting algebra, whenever ${\displaystyle P}$  is an object of ${\displaystyle {\widehat {C}}=\mathbf {Set} ^{C^{\mathrm {op} }}}$  for small ${\displaystyle C}$ .
• For any morphism ${\displaystyle f:X\to Y}$  of ${\displaystyle {\widehat {C}}}$ , the pullback functor of subobjects ${\displaystyle f^{*}:\mathrm {Sub} _{\widehat {C}}(Y)\to \mathrm {Sub} _{\widehat {C}}(X)}$  has a right adjoint, denoted ${\displaystyle \forall _{f}}$ , and a left adjoint, ${\displaystyle \exists _{f}}$ . These are the universal and existential quantifiers.
• A locally small category ${\displaystyle C}$  embeds fully and faithfully into the category ${\displaystyle {\widehat {C}}}$  of set-valued presheaves via the Yoneda embedding which to every object ${\displaystyle A}$  of ${\displaystyle C}$  associates the hom functor ${\displaystyle C(-,A)}$ .
• The category ${\displaystyle {\widehat {C}}}$  admits small limits and small colimits.[2] See limit and colimit of presheaves for further discussion.
• The density theorem states that every presheaf is a colimit of representable presheaves; in fact, ${\displaystyle {\widehat {C}}}$  is the colimit completion of ${\displaystyle C}$  (see #Universal property below.)

## Universal property

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

Proposition[3] — Let C, D be categories and assume D admits small colimits. Then each functor ${\displaystyle \eta :C\to D}$  factorizes as

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

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

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

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

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

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

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

## 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.)[4] 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: ${\displaystyle C\to PShv(C)}$  is fully faithful (here C can be just a simplicial set.)[5]

## Notes

1. ^
2. ^ Kashiwara & Schapira 2005, Corollary 2.4.3.
3. ^ Kashiwara & Schapira 2005, Proposition 2.7.1.
4. ^ Lurie, Definition 1.2.16.1.
5. ^ Lurie, Proposition 5.1.3.1.

## References

• Kashiwara, Masaki; Schapira, Pierre (2005). Categories and sheaves. Grundlehren der mathematischen Wissenschaften. 332. Springer. ISBN 978-3-540-27950-1.
• Lurie, J. Higher Topos Theory.
• Mac Lane, Saunders; Moerdijk, Ieke (1992). Sheaves in Geometry and Logic. Springer. ISBN 0-387-97710-4.