public void Phi2()
    {
        var expected = 10083087720778;
        var actual   = Totient.Phi2(10083087720779);

        Assert.Equal(expected, actual);
    }
    public void PhiPhi2()
    {
        var totient  = new Totient(100);
        var expected = totient.Phi(20);
        var actual   = Totient.Phi2(20);

        Assert.Equal(expected, actual);
    }