The mathematical representation
of the PERM function is given by the following equation:

$\begin{array}{c}PERM(n,r)=\frac{n!}{\left(n-r\right)!}\hfill \end{array}$
with

n ≥ 0,

r ≥ 0, and

n≥

r.

If the expression cannot
be computed, a missing value is returned. For moderately large values,
it is sometimes not possible to compute the PERM function.