Pochhammer
From specialfunctionswiki
The Pochhammer symbol $(a)_n$ is a notation that denotes the "rising factorial" function. It is defined by $$\left\{ \begin{array}{ll} (a)_0 &= 1 \\ (a)_n \equiv a^{\overline{n}} &= \displaystyle\prod_{k=0}^{n-1} a+k=a(a+1)(a+2)\ldots(a+n-1). \end{array} \right.$$
Properties
Sum of reciprocal Pochhammer symbols of a fixed exponent
Notes
We are using this symbol to denote the rising factorial (following the notation used by Abramowitz&Stegun and Mathematica) as opposed to denoting the falling factorial (as Wikipedia does).
