Pearson's Function

$$I\left({{{\chi_s}^2\over \sqrt{2(k-1)}}, {k-3\over 2}}\right... ...le{k-1\over 2}}}\right)\over \Gamma\left({k-1\over 2}\right)},$$

where $\Gamma(x)$ is the Gamma Function.

See also Chi-Squared Test, Gamma Function