Lower bounds for decomposable univariate wild polynomials

A univariate polynomial f over a field is decomposable if it is the composition f=g∘h of two polynomials g and h whose degree is at least 2. The tame case, where the field characteristic p does not divide the degree n of f, is reasonably well understood. The wild case, where p divides n, is more challenging. We present an efficient algorithm for this case that computes a decomposition, if one exists. It works for most but not all inputs, and provides a reasonable lower bound on the number of decomposable polynomials over a finite field. This is a central ingredient in finding a good approximation to this number.