end (category theory)

{{About|the type of transformation|the category of morphisms denoted as End|Endomorphism}}

In category theory, an end of a functor $S:\mathbf{C}^{\mathrm{op}}\times\mathbf{C}\to \mathbf{X}$ is a universal dinatural transformation from an object e of X to S.{{sfnp|Mac Lane|2013}}

More explicitly, this is a pair $(e,\omega)$ , where e is an object of X and $\omega:e\ddot\to S$ is an extranatural transformation such that for every extranatural transformation $\beta : x\ddot\to S$ there exists a unique morphism $h:x\to e$

of X with $\beta_a=\omega_a\circ h$

for every object a of C.

By abuse of language the object e is often called the end of the functor S (forgetting $\omega$ ) and is written

: $e=\int_c^{} S(c,c)\text{ or just }\int_\mathbf{C}^{} S.$

Characterization as limit: If X is complete and C is small, the end can be described as the equalizer in the diagram

: $\int_c S(c, c) \to \prod_{c \in C} S(c, c) \rightrightarrows \prod_{c \to c'} S(c, c'),$

where the first morphism being equalized is induced by $S(c, c) \to S(c, c')$ and the second is induced by $S(c', c') \to S(c, c')$ .

Coend

The definition of the coend of a functor $S:\mathbf{C}^{\mathrm{op}}\times\mathbf{C}\to\mathbf{X}$ is the dual of the definition of an end.

Thus, a coend of S consists of a pair $(d,\zeta)$ , where d is an object of X and $\zeta:S\ddot\to d$

is an extranatural transformation, such that for every extranatural transformation $\gamma:S\ddot\to x$ there exists a unique morphism

$g:d\to x$ of X with $\gamma_a=g\circ\zeta_a$ for every object a of C.

The coend d of the functor S is written

: $d=\int_{}^c S(c,c)\text{ or }\int_{}^\mathbf{C} S.$

Characterization as colimit: Dually, if X is cocomplete and C is small, then the coend can be described as the coequalizer in the diagram

: $\int^c S(c, c) \leftarrow \coprod_{c \in C} S(c, c) \leftleftarrows \coprod_{c \to c'} S(c', c).$

Examples

Natural transformations:

Suppose we have functors $F, G : \mathbf{C} \to \mathbf{X}$ then

: $\mathrm{Hom}_{\mathbf{X}}(F(-), G(-)) : \mathbf{C}^{op} \times \mathbf{C} \to \mathbf{Set}$ .

In this case, the category of sets is complete, so we need only form the equalizer and in this case

: $\int_c \mathrm{Hom}_{\mathbf{X}}(F(c), G(c)) = \mathrm{Nat}(F, G)$

the natural transformations from $F$ to $G$ . Intuitively, a natural transformation from $F$ to $G$ is a morphism from $F(c)$ to $G(c)$ for every $c$ in the category with compatibility conditions. Looking at the equalizer diagram defining the end makes the equivalence clear.

Geometric realizations:

Let $T$ be a simplicial set. That is, $T$ is a functor $\Delta^{\mathrm{op}} \to \mathbf{Set}$ . The discrete topology gives a functor $d:\mathbf{Set} \to \mathbf{Top}$ , where $\mathbf{Top}$ is the category of topological spaces. Moreover, there is a map $\gamma:\Delta \to \mathbf{Top}$ sending the object $[n]$ of $\Delta$ to the standard $n$ -simplex inside $\mathbb{R}^{n+1}$ . Finally there is a functor $\mathbf{Top} \times \mathbf{Top} \to \mathbf{Top}$ that takes the product of two topological spaces.

Define $S$ to be the composition of this product functor with $dT \times \gamma$ . The coend of $S$ is the geometric realization of $T$ .

Notes

References

{{cite book |last1=Mac Lane |first1=Saunders |author-link = Saunders Mac Lane|title=Categories For the Working Mathematician |date=2013 |publisher=Springer Science & Business Media |pages=222–226}}
{{cite arXiv |last1=Loregian |first1=Fosco |title=This is the (co)end, my only (co)friend |eprint=1501.02503|class=math.CT |year=2015 }}

External links

{{nlab|id=end}}

Category:Functors