@Override
    public double rand() {
        double r = MathEx.random();
        return lambda * Math.pow(-Math.log(1 - r), 1 / k);
    }
@Test
    public void testSd() {
        System.out.println("sd");
        WeibullDistribution instance = new WeibullDistribution(1.5, 1.0);
        instance.rand();
        assertEquals(0.61293579, instance.sd(), 1E-7);
    }