Question

What is the cumulative distribution function of a chi-squared distribution?

Answer 1

`F(x;k)=(gamma(k/2,x/2))/(\Gamma(k/2))`

Explanation:

The cumulative distribution function (CDF) of a Chi-squared distribution with `k` degrees of freedom, `\chi^2(k)`, is

`F(x;k)=(gamma(k/2,x/2))/(\Gamma(k/2))`

where `\gamma(s,t)` is the lower incomplete Gamma function and `\Gamma(z)` is the normal Gamma function which are both special functions defined by integrals.

Chi-squared distribution From Wikipedia