Functor

Hom-functor

The Hom-functor is a bifunctor, contravariant in its first argument and covariant in its second, for a locally small category .

On objects, it maps to the Hom-set of morphisms with domain and codomain .

The morphism map for fixed domain is the covariant pushforward

https://q.uiver.app/#q=WzAsNSxbMiwyLCJCIl0sWzIsMCwiQSJdLFs0LDAsIlxcbWF0aHNme0N9KEMsQSkiXSxbNCwyLCJcXG1hdGhzZiBDKEMsQikiXSxbMCwwLCJDIl0sWzEsMCwiZiIsMl0sWzIsMywiXFxtYXRoc2YgQyhDLGYpID0gZl8qIl0sWzQsMCwiZl8qIGciLDJdLFs0LDEsImciXV0=

while the morphism map for fixed codomain is the contravariant pullback

https://q.uiver.app/#q=WzAsNSxbMiwyLCJCIl0sWzIsMCwiQSJdLFs0LDAsIlxcbWF0aHNme0N9KEEsQycpIl0sWzQsMiwiXFxtYXRoc2YgQyhCLEMnKSJdLFswLDAsIkMnIl0sWzEsMCwiZiIsMl0sWzMsMiwiXFxtYXRoc2YgQyhmLEMpID0gZl4qIiwyXSxbMCw0LCJnIl0sWzEsNCwiZl4qIGciLDJdXQ==

Since the following diagram commutes

https://q.uiver.app/#q=WzAsOCxbNCwwLCJcXG1hdGhzZiBDKFgsWSkiXSxbNCwyLCJcXG1hdGhzZiBDKFgsWScpIl0sWzYsMCwiXFxtYXRoc2YgQyhYJyxZKSJdLFs2LDIsIlxcbWF0aHNmIEMoWCcsWScpIl0sWzAsMCwiWCJdLFsyLDAsIlgnIl0sWzAsMiwiWSJdLFsyLDIsIlknIl0sWzAsMiwieF4qIl0sWzAsMSwieV8qIiwyXSxbMSwzLCJ4XioiLDJdLFsyLDMsInlfKiJdLFswLDMsIlxcbWF0aHNmIEMoeCx5KSIsMSx7InN0eWxlIjp7ImJvZHkiOnsibmFtZSI6ImRhc2hlZCJ9fX1dLFs0LDUsIngiXSxbNiw3LCJ5Il0sWzQsNiwiZiIsMix7InN0eWxlIjp7ImJvZHkiOnsibmFtZSI6ImRhc2hlZCJ9fX1dLFs1LDcsInlfKnheKiBmID0geF4qIHlfKiBmIiwxXV0=

this indeed forms a bifunctor.


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