# nLab end

category theory

## Applications

#### Enriched category theory

enriched category theory

## Extra stuff, structure, property

### Homotopical enrichment

#### Limits and colimits

limits and colimits

# Contents

## Disambiguation

This articles is about ends (and coends) in category theory. For ends in topology, see end compactification.

## Idea

An end is a special kind of limit over a functor of the form $F : C^{op} \times C \to D$ (sometimes called a bifunctor).

If we think of such a functor in the sense of profunctors as encoding a left and right action on the object

$\prod_{c \in C} F(c,c)$

then the end of the functor picks out the universal subobject on which the left and right action coincides. Dually, the coend of $F$ is the universal quotient of $\coprod_{c \in C} F(c,c)$ that forces the two actions of $F$ on that object to be equal.

A classical example of an end is the $V$-object of natural transformations between $V$-enriched functors in enriched category theory. Perhaps the most common way in which ends and coends arise is through homs and tensor products of (generalized) modules, and their close cousins, weighted limits and weighted colimits. These concepts are fundamental in enriched category theory.

## Definition

### In ordinary category theory

In ordinary category theory, given a functor $F: C^{op} \times C \to X$, an end of $F$ in $X$ is an object $e$ of $X$ equipped with a universal extranatural transformation from $e$ to $F$. This means that given any extranatural transformation from an object $x$ of $X$ to $F$, there exists a unique map $x \to e$ which respects the extranatural transformations.

In more detail: the end of $F$ is traditionally denoted $\int_{c: C} F(c, c)$, and the components of the universal extranatural transformation,

$\pi_c: \int_{c: C} F(c, c) \to F(c, c)$

are called the projection maps of the end. Then, given any extranatural transformation with components

$\theta_c: x \to F(c, c),$

there exists a unique map $f: x \to e$ such that

$\theta_c = \pi_c f$

for every object $c$ of $C$.

The notion of coend is dual to the notion of end. The coend of $F$ is written $\int^{c: C} F(c, c)$, and comes equipped with a universal extranatural transformation with components

$\iota_c \colon F(c,c) \to \int^{c: C} F(c,c)$

### In enriched category theory

There is a definition of end in enriched category theory, as follows.

#### End of $V$-valued functors

Let $V$ be a symmetric monoidal category, and let $C$ be a $V$-enriched category. Assuming $V$ is also closed monoidal, $V$ may be considered as $V$-enriched; in that case, suppose $F: C^{op} \otimes C \to V$ is a $V$-enriched functor.

Then in particular there is a covariant action of $C$ on $F$, with components

$\lambda_{c, d, e}: F(c, d) \otimes C(d, e) \to F(c, e),$

[where is customary notation for the enriched hom of in ], and a contravariant action of $C$ on $F$, with components

$\rho_{c, d, e}: F(d, e) \otimes C(c, d) \to F(c, e).$

In detail, the covariant action is the adjunct of the morphism

$(F(c,-) : C(d,e) \to [F(c,d), F(c,e)]) \in Hom_V(C(d,e),[F(c,d), F(c,e)])$

$Hom_V(C(d,e),[F(c,d), F(c,e)]) \stackrel{\simeq}{\to} Hom_V(C(d,e)\otimes F(c,d),F(c,e))$

in $V$. Similarly for the contravariant action.

###### Remark

Even if $V$ is not closed monoidal, we can still define a notion of covariant $C$-action, sometimes called a “left” $C$-module, as consisting of a function $F \colon Ob(C) \to Ob(V)$ together with an $Ob(V) \times Ob(V)$-indexed collection of morphisms

$F(c) \times C(c, d) \to F(d)$

satisfying some evident unit and associativity axioms, and regard this notion as a stand-in for the notion of $V$-functor $C \to V$. Similarly we have an evident notion of contravariant $C$-action as a stand-in for a $V$-functor $C^{op} \to V$; notice that we don’t even the symmetry to make sense of this. Finally, we can combine these notions into one of $C$-bimodule, where we have a function $F \colon Ob(C) \times Ob(C) \to Ob(V)$ together with a collection of morphisms

$C(a, b) \otimes F(b, c) \otimes C(c, d) \to F(a, d)$

with evident axioms for a bimodule structure, as a stand-in for a $V$-functor of the form $C^{op} \otimes C \to V$.

A $V$- extranatural transformation

$\theta: v \stackrel{\bullet}{\to} F$

from $v$ to $F$ consists of a family of arrows in $V$,

$\theta_c: v \to F(c, c),$

indexed over objects $c$ of $C$, such that for every pair of objects $(c, d)$ in $V$, the composites of (1) and (2) agree:

$v \otimes C(c, d) \stackrel{\theta_c \otimes 1}{\to} F(c, c) \otimes C(c, d) \stackrel{\lambda_{c, c, d}}{\to} F(c, d) \qquad (1)$
$v \otimes C(c, d) \stackrel{\theta_d \otimes 1}{\to} F(d, d) \otimes C(c, d) \stackrel{\rho_{c, d, d}}{\to} F(c, d) \qquad (2)$

A $V$-enriched end of $F$ is an object $\int_{c: C} F(c, c)$ of $V$ equipped with a $V$-extranatural transformation

$\pi: \int_{c: C} F(c, c) \stackrel{\bullet}{\to} F$

such any $V$-extranatural transformation $\theta$ from $v$ to $F$ is obtained by pulling back the components of $\pi$ along $f: v \to \int_{c: C} F(c, c)$, for some unique map $f$. That is,

$\theta_c = \pi_c f$

for all objects $c$ of $C$.

#### End of $C$-valued functors for $C \in V\Cat$

If $X$ is any $V$-enriched category and $F: C^{op} \otimes C \to X$ is a $V$-enriched functor, then the end of $F$ in $X$ is, if it exists, an object $\int_{c: C} F(c, c)$ of $X$ that represents the functor

$\int_{c: C} X(-,F(c,c))\,.$

That means that the end $\int_{c: C} F(c,c)$ comes equipped with an $Ob(C)$-indexed family of arrows

$\pi_c: I \to X(\int_{c: C} F(c, c), F(c, c))$

in $V$, such that for every object $x$ of $X$, the family of maps

$X(x, \pi_c): X(x, \int_{c: C} F(c, c)) \to X(x, F(c, c))$

are the projection maps realizing $X(x, \int_{c: C} F(c, c))$ as the corresponding end $\int_{c: C} X(x, F(c, c))$ in $V$.

#### End as an equalizer

##### Ordinary ends as equalizers

Now we motivate and define the end in enriched category theory in terms of equalizers.

Recall from the discussion at the end of limit that the limit over an (ordinary, i.e. not enriched) functor

$F : C^{op} \to Set$

is given by the equalizer of

$\prod_{c \in Obj(C)} F(c) \stackrel{\prod_{f \in Mor(c)} (F(f) \circ p_{t(f)}) }{\to} \prod_{f \in Mor(C)} F(s(f))$

and

$\prod_{c \in Obj(C)} F(c) \stackrel{\prod_{f \in Mor(c)} (p_{s(f)}) }{\to} \prod_{f \in Mor(C)} F(s(f)) \,.$

If we want to generalize an expression like this to enriched category theory the explicit indexing over the set of morphisms has to be replaced by something that makes sense in an enriched category.

To that end, observe that we have a canonical isomorphism (of sets, still)

$\prod_{{(c_1 \stackrel{f}{\to} c_2)} \in Mor(C)} F(c_1) \simeq \prod_{c_1,c_2 \in Obj(C)} F(c_1)^{C(c_1,c_2)} \,.$

If we write for the hom-set instead

$[C(c_1,c_2), F(c_1)] := F(c_1)^{C(c_1,c_2)}$

with $[-,-]$ the internal hom in Set, then the expression starts to make sense in any $V$-enriched category.

Still equivalently but suggestively rewriting the above, we now obtain the limit over $F$ as the equalizer of

$\prod_{c \in Obj(C)} F(c) \stackrel{\stackrel{\rho}{\to}}{\stackrel{\lambda}{\to}} \prod_{c_1,c_2 \in Obj(C)} [C(c_1,c_2),F(c_1)] \,,$

where in components

$\rho_{c_1, c_2} : F(c_1) \to [C(c_1,c_2), F(c_1)]$

$C(c_1, c_2) \to * \to [F(c_1), F(c_1)]$

(with the last map the adjunct of $Id_{F(c_1)}$) and where

$\lambda_{c_1, c_2} : F(c_2) \to [C(c_1,c_2), F(c_1)]$

$F_{c_1, c_2} : C(c_1, c_2) \to [F(c_2), F(c_1)] \,.$

So for definiteness, the equalizer we are looking at is that of

$\rho := \prod_{c_1, c_2 \in C} \rho_{c_1,c_2}\circ pr_{F(c_1)}$

and

$\lambda := \prod_{c_1, c_2 \in C} \lambda_{c_1,c_2}\circ pr_{F(c_2)}$

This way of writing the limit clearly suggests that it is more natural to have $\lambda$ and $\rho$ on equal footing. That leads to the following definition.

##### Enriched ends over $V$-valued functors as equalizers

For $V$ a symmetric monoidal category, $C$ a $V$-enriched category and $F : C^{op} \times C \to V$ a $V$-enriched functor, the end of $F$ is the equalizer

(1)$\int_{c \in C} F(c,c) \to \prod_{c \in Obj(C)} F(c,c) \stackrel{\stackrel{\rho}{\to}}{\stackrel{\lambda}{\to}} \prod_{c_1,c_2 \in Obj(C)} [C(c_1,c_2),F(c_1,c_2)]$

with $\rho$ in components given by

$\rho_{c_1, c_2} : F(c_1,c_1) \to [C(c_1,c_2), F(c_1,c_2)]$

$F(c_1,-) : C(c_1, c_2) \to [F(c_1,c_1), F(c_1,c_2)]$

and

$\lambda_{c_1, c_2} : F(c_2,c_2) \to [C(c_1,c_2), F(c_1,c_2)]$

$F(-,c_2) : C(c_1, c_2) \to [F(c_2,c_2), F(c_1,c_2)] \,.$

This definition manifestly exhibits the end as the equalizer of the left and right action encoded by the distributor $F$.

Dually, the coend of $F$ is the coequalizer

(2)$\coprod_{c_1,c_2} C(c_2,c_1) \times F(c_1,c_2)\, \rightrightarrows\, \coprod_c F(c,c)\,\to\, \int^c F(c,c)$

with the parallel morphisms again induced by the two actions of $F$.

#### End as a weighted limit

The end for $V$-functors with values in $V$ serves, among other things, to define weighted limits, and weighted limits in turn define ends of bifunctors with values in more general $V$-categories.

For $C$ and $D$ both $V$-categories and $F : C^\op \times C \to D$ an $V$-enriched functor, the end of $F$ is the weighted limit of $F$

$\int_{c \in C} F(c,c) \coloneqq \{Hom_C, F\} = lim^{Hom_C} F \,,$

with weight $Hom_C : C^{op} \times C \to V$. The coend of $F$ is the colimit

$\int^{c \in C} F(c,c) \coloneqq Hom_{C^{op}} \ast F = \colim^{Hom_{C^{op}}} F$

of $F$ weighted by the hom functor of $C^{op}$.

#### Connecting the two definitions

If $C$ is a $V$-category, then the hom functor $C(-,-) \colon C^{op} \times C \to V$ is the coequalizer in

$\coprod_{c,c'} C(-,c) \times C(c,c') \times C(c',-) \,\rightrightarrows\, \coprod_c C(-,c) \times C(c,-) \,\to\, C(-,-)$

It is also a general fact (see e.g. Kelly, ch. 3) that weighted (co)limits are cocontinuous in their weight: that is,

$\{W \ast V, F\} \cong \{W, \{V-, F\}\}$

and

$(W \ast V) \ast G \cong W \ast (V \ast G)$

This implies that $\{-,F\}$ takes the coequalizer above to an equalizer, which, after some fiddling with the Yoneda lemma, turns out to be isomorphic to (1). Similarly, $(- \ast F)$ takes the analogous coequalizer presentation of $C^{op}(-,-)$ to (2).

## Properties

### $Set$-enriched coends as ordinary colimits

Let the enriching category be $\mathcal{V} =$ Set. We describe a special way in this case to express ends/coends that give weighted limits/colimits in terms of ordinary (co)limits over categories of elements.

Consider

• $C$ a $Set$-enriched category/locally small category tensored over Set;

• $D$ be a small category;

• $F : D \to C$ a functor;

• $W : D^{op} \to Set$ another functor;

• $el W \to D^{op}$ the category of elements of $W$.

###### Proposition

We have a natural isomorphism in $C$

$\int^{d \in D} W(d) \cdot F(d) \simeq \lim_{\to}( (el W)^{op} \to D \stackrel{F}{\to} C )$

between the coend as indicated and the colimit over the opposite of the category of elements of $W$.

This is equation (3.34) in (Kelly) in view of (3.70).

###### Example

If $W = D(-,e)$ is a representable functor, then

$(el W)^{op} = D/e$

is the over category over the representing object $e$. This has a terminal object, namely $(e \stackrel{Id}{\to} e$). Therefore

$\lim_\to( D/e \to D \stackrel{F}{\to} C) \simeq F(e) \,.$

Since this is natural in $e$, the above proposition asserts a natural isomorphism

$F(-) \simeq \int^{k \in D} D(k,-) \cdot F(k) \,.$

This statement is sometimes called the co-Yoneda lemma.

### Commutativity of ends and coends

Ordinary limits commute with each other, if both limits exist separately. The analogous statement does hold for ends and coends. Since there it looks like the commutativity of two integrals, it is called the Fubini theorem for ends (for instance Kelly, p. 29).

###### Definition

(Fubini theorem for ends)

Let $\mathcal{V}$ be a symmetric monoidal category. Let $\mathcal{A}$ and $\mathcal{B}$ be small $\mathcal{V}$-enriched categories.

Let

$T : (\mathcal{A} \otimes \mathcal{B})^{op} \otimes (\mathcal{A} \otimes \mathcal{B}) \to \mathcal{V}$

be a $\mathcal{V}$-enriched functor. Then:

If for all object $B,B' \in \mathcal{B}$ the end $\int_{A \in \mathcal{A}} T(A,B,A,B')$ exists, then

$\int_{(A,B) \in \mathcal{A} \otimes \mathcal{B}} T(A,B,A,B) \simeq \int_{A \in \mathcal{A}} \int_{B \in \mathcal{B}} T(A,B,A,B)$

if either side exists. In particular, since $\mathcal{A} \otimes \mathcal{B} \simeq \mathcal{B} \otimes \mathcal{A}$ this implies that

$\int_{B \in \mathcal{B}} \int_{A \in \mathcal{A}} T(A,B,A,B) \simeq \int_{A \in \mathcal{A}} \int_{B \in \mathcal{B}} T(A,B,A,B)$

if either side exists.

## Examples

### Enriched functor categories

For $C$ and $D$ both $V$-enriched categories, the $V$-enriched functor category $[C,D]$ is the $V$-enriched category whose

• objects are $V$-enriched functors $F : C \to D$;

• hom-objects in $V$ are given by the end-formula $[C,D](F,G) := \int_{c \in C} D(F(c), G(c))$.

For $V = Set$ this reproduces of course the ordinary functor category.

### Kan extension

If the $V$-enriched category $D$ is tensored over $V$, then the (left) Kan extension of a functor $F : C \to D$ along a functor $p : C \to B$ is given by the coend

$Lan F : b \mapsto \int^{c \in C} hom(p(c),b) \cdot F(c) \,.$

See Kan extension for more details.

### Geometric realization

A special case of the example of Kan extension is that of geometric realization.

Very generally, geometric realization is the left Kan extension of a functor $F : C \to D$ along the Yoneda embedding $Y : C \to [C^{op},V]$.

The “geometric realization” of an object $X \in [C,V]$ with respect to $F$ is then the coend

$|X| := \int^{c \in C} F(c) \cdot hom(Y(c),X) \simeq \int^{c \in C} F(c) \cdot X(c) \,,$

where the last step on the right uses the Yoneda lemma.

More specifically, traditionally this is thought of as applying to the case where $C = \Delta$ is the simplex category and where $F : \Delta \to Top$ regards the abstract $n$-simplex as the standard simplex as a topological space.

### Tensor product of functors

If $S : C^\op \to D$ and $T : C \to D$ are functors, their tensor product is the coend

$S \otimes_C T = \int^c S(c) \otimes D(c),$

where the tensor product on the right hand side refers to some monoidal structure on $D$.

homotopycohomologyhomology
$[S^n,-]$$[-,A]$$(-) \otimes A$
category theorycovariant homcontravariant homtensor product
homological algebraExtExtTor
enriched category theoryendendcoend
homotopy theoryderived hom space $\mathbb{R}Hom(S^n,-)$cocycles $\mathbb{R}Hom(-,A)$derived tensor product $(-) \otimes^{\mathbb{L}} A$

## References

The standard reference is

• Max Kelly, Basic concepts in enriched category theory (pdf)

• ends of $V$-valued bifunctors are discussed in section 2.1

• the enriched functor category that they give rise to is discussed in section 2.2;

• enriched weighted limits in terms of enriched functor categories are in section 3.1

• the end of general $V$-enriched functors in terms of weighted limits is in section 3.10 .

• Ends, $n$-Category Café discussion.

• Fosco Loregian, This is the (co)end, my only (co)friend (arXiv).

Revised on November 30, 2015 10:48:33 by Sebastian Conybeare? (130.126.255.209)