Because gcd(ai, p) 1, Theorem 4.7 asserts that the congruence ax —a® (mod p) has a unique solution modulo p. Thus, the theorem holds for n — 1.
Now assume inductively that the theorem is true for polynomials of degree k — 1, and consider the case in which f(x) has degree k. Either the congruence f(x) 0 (mod p) has no solutions (and we are finished), or it has at least one solution, call it a. If f(x) is divided by x —a, the result is