  For a positive integer $n$, let $p(n)$ be the number of ways to express $n$
as a sum of positive integers. In this note, we revisit the derivation of the
Rademacher's convergent series for $p(n)$ in a pedagogical way, with all the
details given. We also derive the leading asymptotic behavior of $p(n)$ when
$n$ approaches infinity. Some numerical results are tabled.
