@Override
    public double variance() {
        return 2.0 * nu2 * nu2 * (nu1 + nu2 - 2) / (nu1 * (nu2 - 2) * (nu2 - 2) * (nu2 - 4));
    }
@Test
    public void testVariance() {
        System.out.println("variance");
        FDistribution instance = new FDistribution(10, 20);
        instance.rand();
        assertEquals(0.4320988, instance.variance(), 1E-7);
    }