I have read, from the question: Irreducibility of polynomials in two variables, that all polynomials f(x)-g(y), where f, g are indecomposable polynomials, and there are no a, b such that g(ax+b)=f(x), are irreducible, unless the degrees of f and g are 7, 11, 13, 15, 21, or 31. Is there an example of the exceptional case in degree 7, with the factorization?