Inverse of the regularized upper incomplete gamma function Q(a z)
InverseGammaRegularized[a, s]
InverseGammaRegularized[2, 0.55]
→ 1.5234732121364183Plot[InverseGammaRegularized[2, s], {s, -1, 1}]
→ -Graphics-D[InverseGammaRegularized[a, s], s]
→ Derivative[0, 1][InverseGammaRegularized][a, s]