Let A be an abelian category and D the category having two objects and only one nonidentity morphism between them.
The functor category A$^D$ is also abelian and it is called an arrow category with objects morphisms in A and morphisms commutative squares.
I cannot see the equivalence between the functor category and the arrow category. I understand arrow category but how it is equivalent to the functor category? Any help would be appreciated!