# Stirling transform

In combinatorial mathematics, the Stirling transform of a sequence { an : n = 1, 2, 3, ... } of numbers is the sequence { bn : n = 1, 2, 3, ... } given by

${\displaystyle b_{n}=\sum _{k=1}^{n}\left\{{\begin{matrix}n\\k\end{matrix}}\right\}a_{k},}$

where ${\displaystyle \left\{{\begin{matrix}n\\k\end{matrix}}\right\}}$ is the Stirling number of the second kind, also denoted S(n,k) (with a capital S), which is the number of partitions of a set of size n into k parts.

The inverse transform is

${\displaystyle a_{n}=\sum _{k=1}^{n}s(n,k)b_{k},}$

where s(n,k) (with a lower-case s) is a Stirling number of the first kind.

Berstein and Sloane (cited below) state "If an is the number of objects in some class with points labeled 1, 2, ..., n (with all labels distinct, i.e. ordinary labeled structures), then bn is the number of objects with points labeled 1, 2, ..., n (with repetitions allowed)."

If

${\displaystyle f(x)=\sum _{n=1}^{\infty }{a_{n} \over n!}x^{n}}$

is a formal power series (note that the lower bound of summation is 1, not 0), and

${\displaystyle g(x)=\sum _{n=1}^{\infty }{b_{n} \over n!}x^{n}}$

with an and bn as above, then

${\displaystyle g(x)=f(e^{x}-1).\,}$