Ramanujan's Identity

$$5 {\phi^5(x^5)\over\phi^6(x)} = \sum_{m=0}^\infty P(5m+4) x^m,$$

where

$$\phi(x)=\prod_{m=1}^\infty (1-x^m)$$

and $P(n)$ is the Partition Function P.

See also Ramanujan's Sum Identity