Fratio (or F) Distribution

The distribution of $\left(X_1/X_2\right)\left({\nu }_2/{\nu }_1\right)$ if $X_1$ is chi-squared with $v_1$ degrees of freedom and $X_2$ is chi-squared with $v_2$ degrees of freedom. The support is $x\ge 0$.

where $I\left(x;a,b\right)=I_x\left(a,b\right)$ is the regularized incomplete Beta function.

Implementation: scipy.stats.f