# $\gcd\left(a+b,\frac{a^p+b^p}{a+b}\right)=1$, or $p$

Let $p$ be prime number ($p\gt2$) and $a,b\in\mathbb Z$ ,$a+b\neq0$ ,$\gcd(a,b)=1$ how to prove that $$\gcd\left(a+b,\frac{a^p+b^p}{a+b}\right)=1~~\text{or}~~ p$$

Thanks in advance .

-
See also math.stackexchange.com/questions/721273/…, math.stackexchange.com/questions/491806/proving-gcd-m-n-1 and other questions linked there. –  Martin Sleziak Mar 22 at 7:04

## 1 Answer

$\rm\displaystyle \left(a\!-\!b,\frac{a^p\!-b^p}{a-b}\right) = (a\!-\!b,\,a^{p-1}\!+\cdots+b^{p-1}) = (a\!-\!b,\color{#C00}{pb^{p-1}}) = (a\!-\!b,p)\:$ by $\rm\:a\equiv b\ \ (mod\ a\!-\!b)\:$ and $\rm\:(a,b) = 1\:\Rightarrow\: (a\!-\!b,b) = (a,b)=1\ \cdots\:\Rightarrow\:(a\!-\!b,b^n) = 1\$ by Euclid's Lemma.

Remark $\$ It's a special case $\rm\ f(x) = x^p,\,\ x = a\ \ (so\ \ \color{#0A0}{f'(b)} = \color{#C00}{pb^{p-1}})\:$ of

Theorem $\rm\displaystyle\quad \frac{f(x)-f(b)}{x-b} \: \equiv\ \color{#0A0}{f\:'(b)}\ \ \ (mod\ \:x\!-\!b)\quad$ for $\rm\ f(x)\in \mathbb Z[x]$

Proof $\$ By Taylor, $\rm\,\ f(x)\ =\ f(b) +\: f\:'(b)\ (x\!-\!b) \,+\, (x\!-\!b)^2\: g(x)\ \$ for some $\rm\ g(x) \in \mathbb Z[x]$

-
Note: this is meant as a (big) hint, along with some conceptual background for further motivation. If you have any trouble filling in the details then please feel welcome to ask questions in comments, and I will gladly elaborate. –  Math Gems Mar 25 '13 at 20:05