This distribution is implemented in the Wolfram Language as InverseChiSquareDistribution[nu].
Inverse Chi-Squared Distribution
See also
Chi-Squared DistributionCite this as:
Weisstein, Eric W. "Inverse Chi-Squared Distribution." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/InverseChi-SquaredDistribution.html