for f, k being positiveNat holds ( f = k ! iff ex j, h, w being Nat ex n, p, q, z being positiveNat st ( q =((w * z)+ h)+ j & z =(f *(h + j))+ h & ((((2 * k)|^ 3)*((2 * k)+ 2))*((n + 1)|^ 2))+ 1 is square & p =(n + 1)|^ k & q =(p + 1)|^ n & z = p |^(k + 1) ) )