public StudentTDistribution(final double degFreedom, final RandomEngine engine) {
Validate.isTrue(degFreedom > 0, "degrees of freedom");
Validate.notNull(engine);
_degFreedom = degFreedom;
_dist = new StudentT(degFreedom, engine);
_beta = new InverseIncompleteBetaFunction(degFreedom / 2., 0.5);
}