<s>[INST] generating function formula Suppose $\lambda$  is a young tableau, with one color boxes. 
Prove the generating function, 
$F(x) = \Sigma_{\lambda} x^{i} = \prod_{n \geq 1} \frac{1}{1-x^n} $, where $i$ is the number of boxes in the young tableau. 
attempt: 
$\Sigma_{\lambda} x^{i} = \prod_{n \geq 1} \frac{1}{1-x^n} =  \frac{1}{1-x} \cdot \frac{1}{1-x^2} \cdot \frac{1}{1-x^3} \cdot \frac{1}{1-x^4}\cdots = (1 + x + x^2 + x^3 + \cdots)(1 + x^2 + x^4+ \cdots )(1 + x^3 + x^6 + x^9 + \cdots)( 1+ x^4 + x^8 + x^{12} \cdots ) = 1 + x + 2x^2 + 3x^3 + \cdots$ .
Here this one comes from when we have zero boxes so we would have $x^0$, and when there is only one box, then we have $x^1 $, similarly, there are $2x^2$ since there are two ways of having a young tableau with two boxes (horizontally, and vertically), and so on.
Can someone please help me?I am stuck after this. I would really appreciate it.
Thank you.
 [/INST] Suppose we have a Young tableaux with shape
\begin{eqnarray*}
(\underbrace{k,\cdots,k}_{a_k \text{times}}, \underbrace{k-1,\cdots,k-1}_{a_{k-1} \text{times}},\cdots,\underbrace{2,\cdots,2}_{a_2 \text{times}}\underbrace{1,\cdots,1}_{a_1 \text{times}})
\end{eqnarray*}
This will give a contribution of $x^n$  to the generating function, where $n=a_1+2a_2+\cdots+(k-1)a_{k-1}+ka_k$.
To get all possible Young tableauxs, $k$ can take any value $1,2,\cdots$ and the values $a_k$ can take any value $0,1,\cdots$. So the generating function is given by
\begin{eqnarray*}
\sum_{a_1=0}^{\infty} \sum_{a_2=0}^{\infty} \cdots \sum_{a_{k-1}=0}^{\infty} \sum_{a_k=0}^{\infty} \cdots x^n
\end{eqnarray*}
All of the sums will "decouple" and this can be rewritten as 
\begin{eqnarray*}
\sum_{a_1=0}^{\infty} x^{a_1} \sum_{a_2=0}^{\infty} x^{2 a_2}\cdots \sum_{a_{k-1}=0}^{\infty} x^{(k-1)a_{k-1}} \sum_{a_k=0}^{\infty} x^{ka_k}\cdots 
\end{eqnarray*}
Now sum each of these geometric plums & we get
\begin{eqnarray*}
\frac{1}{(1-x)} \frac{1}{(1-x^2)} \cdots \frac{1}{(1-x^{k-1})} \frac{1}{(1-x^k)} \cdots = \prod_{k=0}^{\infty} \frac{1}{(1-x^k)}
\end{eqnarray*}
</s>