Definition:Stirling Numbers of the First Kind

Definition
Stirling Numbers of the First Kind come in two forms.

In the below, $n$ and $k$ are always non-negative integers.

Unsigned Stirling Numbers of the First Kind
These are defined recursively by:


 * $\displaystyle \left[{n \atop k}\right] = \begin{cases}

\delta_{n k} & : k = 0 \text{ or } n = 0 \\ \left[{n-1 \atop k-1}\right] + \left({n-1}\right) \left[{n-1 \atop k}\right] & : \text{otherwise} \\ \end{cases}$

where $\delta_{nk}$ is the Kronecker delta.

Signed Stirling Numbers of the First Kind
These are defined recursively by:


 * $s \left({n, k}\right) = \begin{cases}

\delta_{n k} & : k = 0 \text{ or } n = 0 \\ s \left({n - 1, k - 1}\right) - \left({n - 1}\right) s \left({n - 1, k}\right) & : \text{otherwise} \\ \end{cases}$

where $\delta_{nk}$ is the Kronecker delta.

Also see

 * Definition:Stirling's Triangles

Compare with

 * Definition:Stirling Numbers of the Second Kind
 * Definition:Pascal's Triangle

Notation
The notation given here for the unsigned type is that proposed by and publicised by.

The notation given here for the signed type is similar to alternative versions of the unsigned. Usage is inconsistent in the literature.