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How to show that

$$ \gcd\bigg( {a^n-b^n \over a-b} ,a-b\bigg )=\gcd(n d^{n-1},a-b ) $$ $a,b\in \mathbb Z$

where $d=\gcd(a,b)$?

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2  
See here for a very simple proof. –  Bill Dubuque Feb 8 at 2:53

3 Answers 3

up vote 2 down vote accepted

We have $\large\ d=(a,b)\ ,\ $ thus $\large\ \exists\ A,B\ \ \ a=Ad,\ b=Bd,\ (A,B)=1$

$\large\left(\LARGE\frac{a^n-b^n}{a-b}\large,a-b\right)=(n d^{n-1},a-b)$
$\large\ d\left(d^{n-2}\cdot\LARGE\frac{A^{\ n}-B^{\ n}}{A-B}\large,A-B\right)=d(n d^{n-2},A-B)$

Let $\large\ m=A-B\ ,\ \ \ \ $ then $\large\ (m,B)=1$

$\large\ \left(d^{n-2}\cdot\LARGE\frac{(B+m)^n-B^n}{m}\large,m\right)=(nd^{n-2},m)$
$\large\ \left(d^{n-2}\cdot(nB^{n-1}+Qm),m\right)=(nd^{n-2},m)\ \ \ \ $ for some integer Q
$\large\ \left(nd^{n-2}B^{n-1},m\right)=(nd^{n-2},m)\ ,\ \ $ which is due to $\large (m,B)=1$.

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thanks for help. I don't know line 5 to 6 ? –  World Nov 29 '12 at 11:14
    
@World - See Binomial Expansion –  Egor Skriptunoff Nov 29 '12 at 11:46
    
1)mQ is $m \Big( {n \choose 2} B^{n-2} + ... \Big)$ ? 2) how to remove Qm from expression $\large\ \left(d^{n-2}\cdot(nB^{n-1}+Qm),m\right)=(nd^{n-2},m)$ –  World Nov 29 '12 at 12:17
    
@World - 1) Yes, 2) Using (x,y)=(x+qy,y) –  Egor Skriptunoff Nov 29 '12 at 12:26

Putting $c=a-b,$ we get, $$(a-b, \frac{a^n-b^n}{a-b})=(c, \frac{(b+c)^n-b^n}c)=(c,\binom n 1 b^{n-1}+\binom n 2 b^{n-2}c+\cdots+c^{n-1})=(c,nb^{n-1})$$

As $(c,b)=(a-b,b)=(a,b)=d,$ let $\frac c C=\frac b B=d$ so that $(B,C)=1$

$$(c,nb^{n-1})=(Cd,nB^{n-1}d^{n-1})=d(C,nB^{n-1}d^{n-2})=d(C,nd^{n-2})$$ as $(B,C)=1$

$$(c,nb^{n-1})=d(C,nd^{n-2})=(Cd,nd^{n-1})=(c,nd^{n-1})=(a-b, nd^{n-1})$$

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This answer does the case $\gcd(a,b)=1$ only.

**Main idea:**$\let\v\nu\let\geq\geqslant$ investigate how many times a prime divides $a^n-b^n$.

For $m\in\mathbb N$, let $\v_p(m)$ denote the exponent (multiplicity) of the prime $p$ in the prime factorisation of $m$.

Lemma (Lifting The Exponent) let $a,b\in\mathbb Z$, $p\mid a-b$ prime, $p\nmid a$ and $n\in\mathbb N$.

  1. If $p\nmid n$, then $$\v_p(a^n-b^n)=\v_p(a-b).\tag{1}$$

  2. If $p$ is odd, then $$\v_p(a^n-b^n)=\v_p(a-b)+\v_p(n).\tag{2}$$

  3. If $p=2$ and $4\mid a-b$, then $$\v_2(a^n-b^n)=\v_2(a-b)+\v_2(n).\tag{3}$$

  4. If $p=2$ and $2\mid n$, then $$\v_2(a^n-b^n)=\v_2(a^2-b^2)+\v_2(\tfrac n2).\tag{4}$$

Proof. See here (Lemma 1, Theorem 1, Theorem 2, Theorem 4)
Note that 1. is contained in 2. and 3. except when $p=2$ and $4\nmid a-b$.
4. is an easy consequence of 3. by letting $a\mapsto a^2$, $b\mapsto b^2$ and $n\mapsto\frac n2$.

Let $p$ be a prime divisor of $a-b$. Because $\gcd(a,b)=1$, we have $p\nmid a$. We'll prove that $$\v_p\left(\gcd\left(\frac{a^n-b^n}{a-b},a-b\right)\right)=\v_p(\gcd(n,a-b)).$$

If $p$ is odd or $p=2$ and $4\mid a-b$, then $\v_p\left(\frac{a^n-b^n}{a-b}\right)=\v_p(n)$ by $(2),(3)$, hence

$$\begin{align*}\v_p\left(\gcd\left(\frac{a^n-b^n}{a-b},a-b\right)\right)&=\min\left(\v_p\left(\frac{a^n-b^n}{a-b}\right),\v_p(a-b)\right)\\ &=\min(\v_p(n),\v_p(a-b))\\ &=\v_p(\gcd(n,a-b)).\end{align*}$$

The only case left is $p=2$ and $4\nmid a-b$.
If $n$ is odd, then by $(1)$ we know that $\frac{a^n-b^n}{a-b}$ is odd too.
If $n$ is even, $(4)$ gives that $\v_p(a^n-b^n)\geq\v_p(a-b)+1$, hence $\frac{a^n-b^n}{a-b}$ is even.

Either way, $$\v_2\left(\gcd\left(\frac{a^n-b^n}{a-b},a-b\right)\right)=\v_2(\gcd(n,a-b)).$$

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