Euler's Pentagonal Numbers Theorem

Theorem
Consider the infinite product:
 * $\ds P = \prod_{n \mathop \in \Z_{>0} } \paren {1 - x^n}$

Then $P$ can be expressed as:
 * $\ds P = \sum_{n \mathop \in \Z_{>0} } \paren {-1}^{\ceiling {n / 2} } x^{GP_n}$

where:
 * $\ceiling {n / 2}$ denotes the ceiling of $n / 2$
 * $GP_n$ denotes the $n$th generalized pentagonal number.

That is:
 * $P = 1 - x - x^2 + x^5 + x^7 - x^{12} - x^{15} + \cdots$