Compositions and collisions at degree p2

A univariate polynomial f over a field is decomposable if [email protected]?h=g(h) for nonlinear polynomials g and h. In order to count the decomposables, one wants to know, under a suitable normalization, the number of equal-degree collisions of the form [email protected]?h=g^@[email protected]?h^@? with (g,h) (g^@?,h^@?) and degg=degg^@?. Such collisions only occur in the wild case, where the field characteristic p divides degf. Reasonable bounds on the number of decomposables over a finite field are known, but they are less sharp in the wild case, in particular for degree p^2. We provide a classification of all polynomials of degree p^2 with a collision. It yields the exact number of decomposable polynomials of degree p^2 over a finite field of characteristic p. We also present an efficient algorithm that determines whether a given polynomial of degree p^2 has a collision or not.