Partition function (number theory)

A partition of a positive integer $n$ is a decomposition of $n$ as a sum of positive integers. For example, the partitions of 4 read: $4, 3+1, 2+2, 2+1+1, 1+1+1+1$. The partition function $p(n)$ counts the number of different partitions of $n$, so that $p(4) = 5$. L. Euler gave a non-trivial recurrence relation for $p(n)$ (see [a1]) and Ramanujan discovered the surprising congruences $p(5m+4) \equiv 0 \pmod 5$, $p(7m+5) \equiv 0 \pmod 7$, $p(11m+6) \equiv 0 \pmod{11}$, and others. He also found the asymptotic relation $$p(n) \sim \frac{e^{K \sqrt{n}}}{4n\sqrt{3}}\ \ \text{as}\ \ n \rightarrow \infty \ ,$$ where $K = \pi\sqrt{2/3}$. Later this was completed to an exact series expansion by H. Rademacher (see [a2]).