Dirichlet series generating function I am stuck on how to do this question:
Let d(n) denote the number of divisors of n. Show that the dirichlet series generating function of the sequence {(d(n))^2} equals C^4 (s)/ C(2s).  
C(s) represents the riemann zeta function, I apologize, I am not very accustomed with LaTex. Any help is highly appreciated, I am studying for an exam. Everywhere I have looked on the internet says it is obvious, but none of them seem to want to explain why or how to do it, so please help. thank you

There are many different ways to approach this, depending on what you are permitted to use. One simple way is to use Euler products.
The Euler product for $$Q(s) = \sum_{n\ge 1} \frac{d(n)^2}{n^s}$$ is given by
$$ Q(s) = \prod_p \left( 1 + \frac{2^2}{p^s} + \frac{3^2}{p^{2s}} 
+  \frac{4^2}{p^{3s}} + \cdots \right).$$
This should follow by inspection considering that $d(n)$ is multiplicative and $d(p^v) = v+1$, with $p$ prime.
Now note that $$\sum_{k\ge 0} (k+1)^2 z^k = 
\sum_{k\ge 0} (k+2)(k+1) z^k - \sum_{k\ge 0} (k+1) z^k \\=
\left(\frac{1}{1-z}\right)'' - \left(\frac{1}{1-z}\right)' =
\frac{1+z}{(1-z)^3}.$$
It follows that the Euler product for $Q(s)$ is equal to
$$ Q(s) = \prod_p \frac{1+1/p^s}{(1-1/p^s)^3}.$$
On the other hand, we have
$$ \zeta(s) = \prod_p \frac{1}{1-1/p^s}$$ so that
$$ \frac{\zeta^4(s)}{\zeta(2s)} =
\prod_p \frac{\left(\frac{1}{1-1/p^s}\right)^4}{\frac{1}{1-1/p^{2s}}} =
\prod_p \frac{1-1/p^{2s}}{\left(1-1/p^s\right)^4} =
\prod_p \frac{1+1/p^s}{\left(1-1/p^s\right)^3}.$$
The two Euler products are the same, QED.
