About non central chi-square, I have read paper from Castano-Martinez and Lopez-Blazquez(2005). They made expansion of $\chi_n^2(\delta)$ using inverse-Laplace transform and used Laguerre polynomial. I have a little confusion about its notation.
In page 4, equation 2.3, the probability density function of $ \chi_n^2(\delta)$ is:
The one that confuses me is subscript $k$ in denominator expression $(n/2)_k$ (after $\sum$). What does $k$ means? is it a power or index?