Understanding adjoints has always been (and continues to be) a bit of a struggle for me.

Today I stumbled upon a property of adjoint functors which seemed extremely intuitive to me. I was wondering why this property isn't mentioned more often in introductory category theory literature, and whether or not it completely characterizes adjunctions.

If two functors $F:C\to D$ and $U:D\to C$ are adjoint $F\dashv U$, then for every $f:F(Y)\to X$ in $D$ there exists an $\hat f:Y\to U(X)$ in $C$ such that

$$ U(f)\circ \eta_Y = \hat f$$ $$ \epsilon_X\circ F(\hat f)=f$$

If we substitute the top equation into the bottom, we get

$$ \epsilon_X\circ F(U(f)\circ \eta_Y)=f$$

and by functoriality we get

$$ \epsilon_X\circ F(U(f))\circ F(\eta_Y)=f$$ $$ \epsilon_X\circ (F\circ U)(f)\circ F(\eta_Y)=f$$

What the last equation says is that we can recover any morphism $f$ from the action of the "round trip endofunctor" $F\circ U$ on it by pre-composing with $\epsilon_X$ and post-composing with $F(\eta_Y)$. These two morphisms are determined only by the domain and codomain of $f$ -- we only needed to know $X$ and $F(Y)$ in order to pick the two morphisms. We would have picked the same two morphisms for some $g\neq f$ as long as $g:F(Y)\to X$.

So, I believe it is correct to say that "if the domain of a morphism is within the range of a functor which has a right adjoint, then it can be recovered from the action of the composite endofunctor on it by pre-composition with some morphism and post-composition with some other morphism, where the choice of these two morphisms is completely determined by the domain and codomain of the original morphism". There is, of course, an equivalent statement for morphisms with a codomain in the range of a functor with a left adjoint.

So, my three questions are: (1) is this correct, (2) if so, why isn't it used to explain adjunctions to beginners (I certainly would have caught on quicker!) and (3) does the condition completely characterize adjoint functors?

Thanks,

recoveredfrom FU(f), by pre- and post-composing ..."? – Charles Rezk Mar 23 '10 at 20:23triangular identities$U\varepsilon\circ\eta U=I_U$, $\varepsilon F\circ F\eta=I_F$, see part (v) of Theorem IV.2 on page 83 of Mac Lane. To get the triangular identities from yours, just substitute $f=1_{FY}$ (so that $X=FY$), and similarly in the dual. BTW, the derivation of the triangular identities is very similar to yours, see p. 82 in Mac Lane. – user2734 Mar 23 '10 at 22:55