Composition of adjoint functors

Question

Does the composition of adjoint functors again form an adjunction?

Say $\langle F_1,G^1\rangle$ is an adjoint pair between two categories A and B and $\langle F_2,G^2\rangle$ is also an adjoint pair between the categories B and C, then would $\langle F_2\circ F_1,G^1\circ G^2\rangle$ be an adjoint pair between the categories A and C?

If not is there some way of obtain an adjunction given the assumptions made on the 4 functors, or by adding extra properties to the categories?

Woops.. I recopied the wrong thing from my note pad :0 – AB_IM Feb 19 '14 at 14:29 — AB_IM, Feb 19 '14 at 14:29
Ok. What have you tried? The proof is one line long. – Martin Brandenburg Feb 19 '14 at 14:37 — Martin Brandenburg, Feb 19 '14 at 14:37

score 8 · Accepted Answer · edited Dec 30 '21 at 19:33

8

Let $a$ be an object in $A$, $c$ be an object in $C$ and $b$, $\tilde{b}$ be objects in $B$.

The adjunctions hypothesized give: $C(F_2(b),c)\cong B(b,G^2(c))$ and $B(F_1(a),\tilde{b}) \cong A(a,G^1(\tilde{b}))$.
Now setting $b:=F_1(a)$ and $\tilde{b}:=G^2(c)$ it may be concluded: $A(a,G^1(G^2(c))) \cong B(F_1(a),G^2(c)) \cong C(F_2(F_1(a)),c)$.

So $F_2\circ F_1$ is left adjoint to $G^1\circ G^2$.

edited Dec 30 '21 at 19:33

Elías Guisado Villalgordo

5,265

answered Feb 19 '14 at 14:51

AB_IM

6,978

4

I guess I should stop being afraid of my own solutions :P Thanks Martin :) – AB_IM Feb 19 '14 at 14:52
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Also note: a composition of natural isomorphisms is still natural. – Kenta S Jan 09 '22 at 15:21

Composition of adjoint functors

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