BetaRegularized[z, a, b] gives the regularized incomplete beta function .
Mathematical function (see Section A.3.10).
For non-singular cases, .
BetaRegularized[, , a, b] gives the generalized regularized incomplete beta function defined in non-singular cases as Beta[, , a, b]/Beta[a, b].
Note that the arguments in BetaRegularized are arranged differently from those in GammaRegularized.
See The Mathematica Book: Section 3.2.10.
See also: Beta, InverseBetaRegularized.