# Help with proof.

• July 18th 2011, 01:08 PM
Cairo
Help with proof.
How can you prove that

$\prod_{n=1}^{\infty}(1-x^{n})^{\mu(n)/n}=e^{-x}$

for $|x|<1$.

Here, $\mu(n)$ is the Mobius function.

• July 18th 2011, 08:33 PM
Drexel28
Re: Help with proof.
Quote:

Originally Posted by Cairo
How can you prove that

$\overset{\infty}{\underset{n=1}{\prod}}(1-x^{n})^{\mu(n)/n}=e^{-x}$

for $|x|<1$.

Here, $\mu(n)$ is the Mobius function.

What have you tried? Begin by taking the logarithm of both sides to get that, if $F(x)$ is our function, $\displaystyle \log(F(x))=\sum_{n=1}^{\infty}\frac{\mu(n)}{n}\log \left(1-x^n\right)=-\sum_{n=1}^{\infty}\sum_{m=1}^{\infty}\frac{\mu(n) }{n}\frac{x^{mn}}{m}$. But show that the coefficient of $x^n$ in this is $\displaystyle \frac{-1}{n}\sum_{d\mid n}\mu(d)$ which I'm sure you know is $-1$ if $n=1$ and $0$ otherwise. Thus, $\log(F(x))=-x$.
• July 18th 2011, 10:49 PM
Cairo
Re: Help with proof.
Thanks for this, Drexel.

I was going down a very different road and trying to solve a first order differential equation by considering generating functions.

Your approach looks much more easier to prove.

(Rofl)