The sequence is as follows:
..., -6, -2, 0, 0, 2, 6, 12, 20, ...
As you can see, the difference between the numbers is a linear progression. I'm having troubles generalizing the sequence though :S
When I include the negatives into the sequence wolfram returns this function:
(-2 x^3/(x - 1)^3) - 2 x - 6
but when I graph this function it doesn't appear to resemble the sequence at all :S Can someone please explain why?