The build in function
BetaRegularized gives me an answer for the following values BetaRegularized[1/2 I, 1/3, 1/3]; namely 0.36 + 0.25 I.
But the InverseBetaRegularized[0.36 + 0.25 I, 1/3, 1/3] won't give me a numerical answer...
Does anybody know why?(It works perfect for the case when the first argument is real)