Lifting The Exponent Lemma/Lemma

Lemma
Let $x, y \in \Z$ be integers.

Let $p$ be an odd prime.

Let:
 * $p \mathrel \backslash x - y$

and:
 * $p \nmid x y$.

Then
 * $\nu_p \left({x^p - y^p}\right) = \nu_p \left({x - y}\right) + 1$

where $\nu_p$ denotes $p$-adic valuation.

Proof
Let $\nu_p \left({x - y}\right)=k$.

Then $x=p^k m + y$ where $p \nmid m$.

We have:

Note that all terms in the above expression have a factor of $p$ to the order at least $k+1$.

So, $p^{k+1} \mid x^p - y^p$.

Also note that all terms in the summation have a factor of $p$ to the order at least $k+2$.

But in the term $p^{k+1} m y^{p-1}$, since $p \nmid m$ and $p \nmid y$,

we have $p^{k+2} \nmid p^{k+1} m y^{p-1}$.

So, $p^{k+2} \nmid x^p - y^p$.

So by definition of $p$-adic valuation, $\nu_p \left({x^p - y^p}\right)=k+1$.