I was surprised to learn of Durfee squares, which can be visually explained as the largest square contained within a partition's Ferrers diagram.
Moreover, partition identities have always amused me by how complicated they appear at first to prove. Out of curiosity, can Durfee squares be used to prove either of the following partition identities?
$ \prod_{i\geq 1}(1-qx^i)^{-1}=\sum_{j\geq 0}\frac{x^{j^2}q^j}{(1-x)\cdots(1-x^j)(1-qx)\cdots(1-qx^j)} $ or $ \prod_{i\geq 1}(1+qx^{2i-1})=\sum_{j\geq 0}\frac{x^{j^2}q^j}{(1-x^2)(1-x^4)\cdots(1-x^{2j})}? $
In fact, in Andrews and Eriksson's Integer Partitions, Exercise 101 on page 77 suggests that Durfee squares can be used to determine these formulas. How can this method be employed? Thanks.