End (category theory)

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In category theory, an end of a functor is a universal dinatural transformation from an object e of X to S.[1]

More explicitly, this is a pair , where e is an object of X and is an extranatural transformation such that for every extranatural transformation there exists a unique morphism of X with for every object a of C.

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

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

where the first morphism being equalized is induced by and the second is induced by .

Coend

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The definition of the coend of a functor   is the dual of the definition of an end.

Thus, a coend of S consists of a pair  , where d is an object of X and   is an extranatural transformation, such that for every extranatural transformation   there exists a unique morphism   of X with   for every object a of C.

The coend d of the functor S is written

 

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

 

Examples

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  • Natural transformations:

    Suppose we have functors   then

     .

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

     

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

  • Geometric realizations:

    Let   be a simplicial set. That is,   is a functor  . The discrete topology gives a functor  , where   is the category of topological spaces. Moreover, there is a map   sending the object   of   to the standard  -simplex inside  . Finally there is a functor   that takes the product of two topological spaces.

    Define   to be the composition of this product functor with  . The coend of   is the geometric realization of  .

Notes

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References

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  • 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].
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