End (category theory)

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

Thus, a coend of S consists of a pair ${\displaystyle (d,\zeta )}$, where d is an object of X and ${\displaystyle \zeta :S{\ddot {\to }}d}$ is an extranatural transformation, such that for every extranatural transformation ${\displaystyle \gamma :S{\ddot {\to }}x}$ there exists a unique morphism ${\displaystyle g:d\to x}$ of X with ${\displaystyle \gamma _{a}=g\circ \zeta _{a}}$ for every object a of C.

The coend d of the functor S is written

${\displaystyle d=\int _{}^{c}S(c,c){\text{ or }}\int _{}^{\mathbf {C} }S.}$

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

${\displaystyle \int ^{c}S(c,c)\leftarrow \coprod _{c\in C}S(c,c)\leftleftarrows \coprod _{c\to c'}S(c',c).}$

• Natural transformations:

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

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

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

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

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

• Geometric realizations:

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

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

• Mac Lane, Saunders (2013). Categories For the Working Mathematician. Springer Science & Business Media. pp. 222–226.
• Loregian, Fosco (2015). "This is the (co)end, my only (co)friend". arXiv:1501.02503 [math.CT].

• end at the nLab