Wolstenholme's theorem states that for a prime $p>3$
and
$$ \frac{1}{1^2}+\frac{1}{2^2}+\frac{1}{3^2}+……+\frac{1}{(p-1)^2}=\frac{a}{b}.$$
$ a\equiv 0\pmod p $
I think that I found a reasonable way to prove this theorem but I face a small obstacle at the end of the proof.
My solution steps are as follows.
- make common factor for the fractions.
- for each term in the numerator take (mod p) and simplify the result using the multiplicative inverse in (mod p)
for example the first term in numerator is
$ 2^2 \times 3^2 \times ….. \times (p-1)^2 $
in this quantity each term has its multiplicative inverse (mod p) multiplied by it so this quantity is only $ \equiv 1\pmod p $.
and after doing the same thing to other terms you get at the end that
the numerator is $ \equiv (1^2+1^2+2^2+3^2+….(p-2)^2)\pmod p $ .
after using the formula for summing n squared numbers and simplifying I got
$$a \equiv \frac{2p^3-9p^2+13p}{6}\pmod p $$
this is the question :
for p>3 (p is a prime number)
how to prove that this quantity :
$$ \frac{2p^3-9p^2+13p}{6} $$ is divisible by p .
or equivalently
$$ \frac{2p^2-9p+13}{6} $$ is Integer ( belongs to Z ) .
I appreciate any attempts for help.
Best Answer
HINTS
In your numerator, $2p^2$ is always even and the other terms are always odd, and the difference of odd terms is always even, so the numerator is always even.
Since $p$ is a prime, we cannot have $p \equiv 0 \pmod{3}$. Thus, $p = 3n \pm 1$. The middle term clearly is divisible by $3$ and the corner ones yield $$ \begin{split} 2p^2+13 &= 2(3n\pm 1)^2+13\\ &= 2\left(9n^2+6n+1\right)+13\\ &= 18n^2+12n+15, \end{split} $$ and all coefficients are divisible by 3.
Can you finish this?