A function between categories which maps objects to objects and morphisms to morphisms. Functors exist in both covariant and contravariant types.
See alsoCategory, Contravariant Functor, Covariant Functor, Eilenberg-Steenrod Axioms, Forgetful Functor, Morphism, Schur Functor, Tensor Product Functor
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Weisstein, Eric W. "Functor." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Functor.html