Wilson's Theorem
Statement
If and only if p is a prime, then is a multiple of p. Written more mathematically,
Proof
Wilson's theorem is easily verifiable for 2 and 3, so let's consider . If
is composite, then its positive factors are among
Hence,
, so
.
However ifis prime, then each of the above integers are relatively prime to
. So for each of these integers
there is another
such that
. It is important to note that this
is unique modulo
, and that since
is prime,
if and only if
is
or
. Now if we omit
and
, then the others can be grouped into pairs whose product is congruent to one,
Finally, multiply this equality by p-1 to complete the proof.
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