Fermat's "biggest", and also his "last" theorem states that xn
+ yn = zn has no solutions in positive integers
x, y, z with n > 2. This has finally been proven by Wiles
in 1995. Here we are concerned with his "little" but perhaps his most
used theorem which he stated in a letter to Fre'nicle on 18 October 1640:
Fermat's Little Theorem.
Let p be a prime which does not divide the integer a,
then ap-1 = 1 (mod p).
As usual Fermat did not provide a proof (this time saying "I would send
you the demonstration, if I did not fear its being too long"
[Burton80, p79]).
Euler first published a proof in 1736, but Leibniz left virtually
the same proof in an unpublished manuscript from sometime before 1683.
Proof.
Start by listing the first p-1 positive multiples of
a:
a, 2a, 3a, ... (p -1)a
Suppose that ra and sa are the
same modulo
p, then we have r = s (mod p), so the
p-1 multiples of a above are distinct and nonzero; that is,
they must be congruent to 1, 2, 3, ..., p-1 in some order.
Multiply all these congruences together and we find
a.2a.3a.....(p-1)a
= 1.2.3.....(p-1) (mod p)
or better,
a(p-1)(p-1)! = (p-1)! (mod p).
Divide both side by (p-1)! to complete the proof.
Sometimes Fermat's Little Theorem is presented in the following form:
Corollary.
Let p be a prime and
a any integer,
then ap = a (mod p).
Proof.
The result is trival (both sides are zero) if p divides
a. If p does not divide a, then we need only
multiply the congruence in Fermat's Little Theorem by a to
complete the proof.
