Adjunctions
An adjunction consists of two functors F:C⇆D:G alongside a bijection
D(F(c),d)≅C(c,G(d))
for all c,d which is natural in both c and d. For this we right F⊣G and say that F is left-adjoint to G.
Typically free and forgetful functors are adjoint. For instance, let U:Grp→Set be the forgetful functor and let F:Set→Grp be the free functor taking a set to the free group over that set.
For a set X, an element w∈F(X) is given by a formal ‘word’
w=w1e1 w2e2 ⋯ wkek
where each wi is an element of X and each ej is an integer and no two adjacent wi are the same elements of X.
A homomorphism φ:F(X)→H out of a free group then consists of choosing some assignment h(x)∈H for each x∈X; the value of all words in F(X) follow from this assignment:
φ(w1e1 w2e2 ⋯ wkek)=φ(w1)e1 φ(w2)e2 ⋯ φ(wk)ek=h(w1)e1 h(w2)e2 ⋯ h(wk)ek
That is, a group homomorphism φ contains exactly the same information as an assignment h:X→U(H).
This softly-phrased fact strengthens into the existence of a bijection
Grp(F(X),H)≅Set(X,U(H))
which is natural in both X and H. In other words, F is left-adjoint to U!