ath:`a > 0`. :math:`\Gamma` is the gamma function
(`scipy.special.gamma`).

`invgamma` takes ``a`` as a shape parameter for :math:`a`.

`invgamma` is a special case of `gengamma` with ``c=-1``, and it is a
different parameterization of the scaled inverse chi-squared distribution.
Specifically, if the scaled inverse chi-squared distribution is
parameterized with degrees of freedom :math:`\nu` and scaling parameter
:math:`\tau^2`, then it can be modeled using `invgamma` with
``a=`` :math:`\nu/2` and ``scale=`` :math:`\nu \tau^2/2`.

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c