arrow category




Every category CC gives rise to an arrow category Arr(C)Arr(C) such that the objects of Arr(C)Arr(C) are the morphisms (or arrows, hence the name) of CC.


For CC any category, its arrow category Arr(C)Arr(C) is the category such that:

  • an object aa of Arr(C)Arr(C) is a morphism a:a 0a 1a\colon a_0 \to a_1 of CC;
  • a morphism f:abf\colon a \to b of Arr(C)Arr(C) is a commutative square
    a 0 f 0 b 0 a b a 1 f 1 b 1 \array { a_0 & \overset{f_0}\to & b_0 \\ \llap{a}\downarrow & & \rlap{b}\downarrow \\ a_1 & \underset{f_1}\to & b_1 }

    in CC;

  • composition in Arr(C)Arr(C) is given simply by placing commutative squares side by side to get a commutative oblong.

This is isomorphic to the functor category

Arr(C):=Funct(I,C)=[I,C]=C I Arr(C) := Funct(I,C) = [I,C] = C^I

for II the interval category {01}\{0 \to 1\}. Arr(C)Arr(C) is also written [2,C][\mathbf{2},C], C 2C^{\mathbf{2}}, [Δ[1],C][\Delta[1],C], or C Δ[1]C^{\Delta[1]}, since 2\mathbf{2} and Δ[1]\Delta[1] (for the 11-simplex) are common notations for the interval category.


Last revised on June 7, 2021 at 05:29:37. See the history of this page for a list of all contributions to it.