Category of elements

In category theory, a branch of mathematics, the category of elements of a presheaf is a category associated to that presheaf whose objects are the elements of sets in the presheaf. It and its generalization are also known as the Grothendieck construction (named after Alexander Grothendieck) especially in the theory of descent, in the theory of stacks, and in fibred category theory. ^[1]

The Grothendieck construction is an instance of straightening.

Significance

In categorical logic, the construction is used to model the relationship between a type theory and a logic over that type theory, and allows for the translation of concepts from indexed category theory into fibred category theory, such as Lawvere's concept of hyperdoctrine.

The category of elements of a simplicial set is fundamental in simplicial homotopy theory, a branch of algebraic topology. More generally, the category of elements plays a key role in the proof that every weighted colimit can be expressed as an ordinary colimit, which is in turn necessary for the basic results in theory of pointwise left Kan extensions, and the characterization of the presheaf category as the free cocompletion of a category.^{[ citation needed ]} See also Density theorem (category theory) for an example usage.

Motivation

If ${\displaystyle \left\{A_{i}\right\}_{i\in I}}$ is a family of sets indexed by another set, one can form the disjoint union or coproduct

${\displaystyle \coprod _{i\in I}A_{i}}$,

which is the set of all ordered pairs ${\displaystyle (i,a)}$ such that ${\displaystyle a\in A_{i}}$. The disjoint union set is naturally equipped with a "projection" map

${\displaystyle \pi$ :\coprod _{i\in I}A_{i}\to I}

defined by

${\displaystyle \pi (i,a)=i}$.

From the projection ${\displaystyle \pi }$ it is possible to reconstruct the original family of sets ${\displaystyle \left\{A_{i}\right\}_{i\in I}}$ up to a canonical bijection, as for each ${\displaystyle i\in I,A_{i}\cong \pi ^{-1}(\{i\})}$ via the bijection ${\displaystyle a\mapsto (i,a)}$. In this context, for ${\displaystyle i\in I}$, the preimage ${\displaystyle \pi ^{-1}(\{i\})}$ of the singleton set ${\displaystyle \{i\}}$ is called the "fiber" of ${\displaystyle \pi }$ over ${\displaystyle i}$, and any set ${\displaystyle B}$ equipped with a choice of function ${\displaystyle f:B\to I}$ is said to be "fibered" over ${\displaystyle I}$. In this way, the disjoint union construction provides a way of viewing any family of sets indexed by ${\displaystyle I}$ as a set "fibered" over ${\displaystyle I}$, and conversely, for any set ${\displaystyle f:B\to I}$ fibered over ${\displaystyle I}$, we can view it as the disjoint union of the fibers of ${\displaystyle f}$. Jacobs has referred to these two perspectives as "display indexing" and "pointwise indexing". ^[2]

The Grothendieck construction generalizes this to categories. For each category ${\displaystyle {\mathcal {C}}}$, family of categories ${\displaystyle \{F(c)\}_{c\in {\mathcal {C}}}}$ indexed by the objects of ${\displaystyle {\mathcal {C}}}$ in a functorial way, the Grothendieck construction returns a new category ${\displaystyle {\mathcal {E}}}$ fibered over ${\displaystyle {\mathcal {C}}}$ by a functor ${\displaystyle \pi }$ whose fibers are the categories ${\displaystyle \{F(c)\}_{c\in {\mathcal {C}}}}$.

Construction

Let ${\displaystyle C}$ be a category and let ${\displaystyle F:C^{\rm {op}}\to \mathbf {Sets} }$ be a set-valued functor. The category el(F) of elements of F (also denoted ∫_CF) is the category whose:

• Objects are pairs ${\displaystyle (A,a)}$ where ${\displaystyle A\in \mathop {\rm {Ob}} (C)}$ and ${\displaystyle a\in FA}$.
• Morphisms ${\displaystyle (A,a)\to (B,b)}$ are arrows ${\displaystyle f:A\to B}$ of ${\displaystyle C}$ such that ${\displaystyle (Ff)b=a}$.

An equivalent definition is that the category of elements of ${\displaystyle F}$ is the comma category ∗↓F, where ∗ is a singleton (a set with one element).

The category of elements of F is naturally equipped with a projection functor Π: ∫_CF→C that sends an object (A, a) to A, and an arrow (A,a)→(B,b) to its underlying arrow in C.

For small C, this construction can be extended into a functor ∫_C from Ĉ to Cat, the category of small categories. Using the Yoneda lemma one can show that ∫_CP≅y↓P, where y:C→Ĉ is the Yoneda embedding.^{[ citation needed ]} This isomorphism is natural in P and thus the functor ∫_C is naturally isomorphic to y↓–:Ĉ→Cat.

For some applications, it is important to generalize the construction to even a contravariant pseudofunctor ${\displaystyle F}$ (the covariant case is similar). Namely, given ${\displaystyle F}$, define the category ${\displaystyle C_{F}}$, where

• an object is a pair ${\displaystyle (x,a)}$ consisting of an object ${\displaystyle x}$ in ${\displaystyle C}$ and an object ${\displaystyle a}$ in ${\displaystyle F(x)}$,
• a morphism ${\displaystyle {\overline {f}}:(x,a)\to (y,b)}$ consists of ${\displaystyle f:x\to y}$ in ${\displaystyle C}$ and ${\displaystyle \varphi$ :(Ff)b\to a.}
• the composition of ${\displaystyle {\overline {f}}}$ above and ${\displaystyle {\overline {g}}=(g,\psi ):(y,b)\to (z,c)}$ consists of ${\displaystyle g\circ f}$ and ${\displaystyle \varphi \circ (Ff)\psi }$; i.e.,

${\displaystyle F(g\circ f)c\simeq (Ff\circ Fg)c{\overset {(Ff)\psi }{\to }}f^{*}b{\overset {\varphi }{\to }}a.}$

Perhaps it is psychologically helpful to think of ${\displaystyle Ff}$ as the pullback of ${\displaystyle f}$ (i.e., ${\displaystyle Ff=f^{*}}$) and then ${\displaystyle (Ff)b}$ is the pullback of ${\displaystyle b}$ along ${\displaystyle f}$.

Note here the associativity of the composition is a consequence of the fact that the isomorphisms ${\displaystyle F(g\circ f)\simeq (Ff\circ Fg)}$ are coherent.

Example

If ${\displaystyle G}$ is a group, then it can be viewed as a category, ${\displaystyle {\mathcal {C}}_{G},}$ with one object and all morphisms invertible. Let ${\displaystyle F:{\mathcal {C}}_{G}\to \mathbf {Cat} }$ be a functor whose value at the sole object of ${\displaystyle {\mathcal {C}}_{G}}$ is the category ${\displaystyle {\mathcal {C}}_{H},}$ a category representing the group ${\displaystyle H}$ in the same way. The requirement that ${\displaystyle F}$ be a functor is then equivalent to specifying a group homomorphism ${\displaystyle \varphi :G\to \operatorname {Aut} (H),}$ where ${\displaystyle \operatorname {Aut} (H)}$ denotes the group of automorphisms of ${\displaystyle H.}$ Finally, the Grothendieck construction, ${\displaystyle F\rtimes {\mathcal {C}}_{G},}$ results in a category with one object, which can again be viewed as a group, and in this case, the resulting group is (isomorphic to) the semidirect product ${\displaystyle H\rtimes _{\varphi }G.}$

Formulation in ∞-categories

Using the language of ∞-categories, the Grothendieck construction can be stated in the following succint way. Namely, it says there is an equivalence of ∞-categories:

${\displaystyle {\textrm {Fct}}(C^{op},{\textrm {Cat}})\to {\textrm {Cart}}(C)}$

between the functor category and the (2, 1)-category of cartesian fibrations (or fibered categories) over ${\displaystyle C}$. ^[3] Moreover, the equivalence is given by sending the pseudofunctor ${\displaystyle F:C^{op}\to {\textrm {Cat}}}$ to the category ${\displaystyle C_{F}}$ of pairs for ${\displaystyle F}$ (see above) and the opposite direction by taking fibers; i.e., ${\displaystyle \pi }$ is mapped to the pseudofunctor ${\displaystyle X\mapsto \pi ^{-1}(X)}$.

In more details, given a cartesian fibration ${\displaystyle \pi }$, define the pseudofunctor ${\displaystyle F}$ as follows. ^[4] For an object ${\displaystyle x}$, ${\displaystyle F(x)=\pi ^{-1}(x)}$. Next, since ${\displaystyle \pi }$ is a cartesian fibration, for each morphism ${\displaystyle f:x\to y}$ and each object ${\displaystyle {\overline {y}}}$ in ${\displaystyle \pi ^{-1}(y)}$, there is an object ${\displaystyle {\overline {x}}}$ in ${\displaystyle \pi ^{-1}(x)}$ as well as a cartesian morphism ${\displaystyle {\overline {f}}_{\overline {y}}:{\overline {x}}\to {\overline {y}}}$ in ${\displaystyle \pi ^{-1}(f)}$. By the axiom of choice, for each ${\displaystyle {\overline {y}}}$, we thus choose ${\displaystyle (Ff){\overline {y}}}$ in ${\displaystyle \pi ^{-1}(x)}$ as well as a cartesian morphism ${\displaystyle {\overline {f}}:(Ff){\overline {y}}\to {\overline {y}}}$. To simplify the notation, we shall let ${\displaystyle f^{*}=Ff}$. We now make

${\displaystyle f^{*}:\pi ^{-1}(y)\to \pi ^{-1}(x)}$

a functor; i.e., it also sends morphisms. If ${\displaystyle \alpha$ :{\overline {y}}_{1}\to {\overline {y}}_{2}} is a morphism in ${\displaystyle \pi ^{-1}(y)}$, since ${\displaystyle {\overline {f}}:f^{*}{\overline {y}}_{2}\to {\overline {y}}_{2}}$ is cartesian, there is a unique morphism ${\displaystyle f^{*}{\overline {y}}_{1}\to f^{*}{\overline {y}}_{2}}$, which we denote by ${\displaystyle f^{*}\alpha }$, such that ${\displaystyle \alpha \circ {\overline {f}}_{{\overline {y}}_{1}}={\overline {f}}_{{\overline {y}}_{2}}\circ f^{*}\alpha }$. By the uniqueness of choices, we have ${\displaystyle f^{*}(\alpha \circ \beta )=f^{*}\alpha \circ f^{*}\beta }$. Thus, ${\displaystyle Ff=f^{*}}$ is a functor. Hence, ${\displaystyle F:C^{op}\to {\textbf {Cat}}}$ is defined. Finally, we show ${\displaystyle F}$ is a pseudofunctor. Roughly, this is because, even though we made a choice using the axiom of choice, different choices differ by unique isomorphisms. Consequently, the isomorphisms ${\displaystyle F(g\circ f)\simeq (Ff)(Fg)}$ will be coherent.

Notes

1. Mac Lane, Saunders; Moerdijk, Ieke (1994). Sheaves in geometry and logic: a first introduction to topos theory (2., corr. print ed.). New York: Springer. ISBN   9780387977102.
2. Jacobs, Bart (1999). Categorical logic and type theory. Amsterdam Lausanne New York [etc.]: Elsevier. ISBN   0444501703.
3. Khan 2023 , Theorem 3.1.5.
4. Vistoli 2008 , Proposition 3.11.

References

• Mac Lane, Saunders (1998). Categories for the Working Mathematician . Graduate Texts in Mathematics 5 (2nd ed.). Springer-Verlag. ISBN   0-387-98403-8.
• Mac Lane, Saunders; Moerdijk, Ieke (1992). Sheaves in Geometry and Logic. Universitext (corrected ed.). Springer-Verlag. ISBN   0-387-97710-4.
• Peter Johnstone, Sketches of an Elephant (2002)
• Garth Warner: Fibrations and Sheaves, EPrint Collection, University of Washington (2012)
• Khan, Adeel A. (2023), Lectures on Algebraic Stacks (PDF)
• Vistoli, Angelo (September 2, 2008). "Notes on Grothendieck topologies, fibered categories and descent theory" (PDF).

Further reading

• Category of elements at the nLab
• Grothendieck Construction at the nLab
• https://mathoverflow.net/questions/322763/why-is-the-straightening-functor-the-analogue-of-the-grothendieck-construction