public void CubicSum()
{
var start = random.Next(1, 100);
var end = random.Next(start + 1, 200);
var uStart = (uint)start;
var uEnd = (uint)end;
Assert.Equal(Enumerable.Range(1, end).Sum(x => x * x * x), (int)MathX.CubicSum(uEnd));
Assert.Equal(Enumerable.Range(start, end - start + 1).Sum(x => x * x * x), (int)MathX.CubicSum(uStart, uEnd));
Assert.Throws <InvalidOperationException>(() => MathX.CubicSum(uEnd, uStart));
Assert.Equal(8u, MathX.CubicSum(2, 2));
Assert.Equal(225u, MathX.CubicSum(0, 5));
}