I played with the numbers a bit, and if we precede the series with three zeros, so that it is 0, 0, 0, 1, 2, 5, 12, ....

Then, it seems to follow that for X>4, F(X) = 2*F(X-1)+F(X-2)-F(X-4).

2 = 2*1 + 0 - 0

5 = 2*2 + 1 - 0

12 = 2*5 + 2 - 0

28 = 2*12 + 5 - 1

66 = 2*28 + 12 - 2

etc....

The next four terms would be:

**11076
**

26013

61094

143485
-- Corrected/edited formula variable. Answer remains the same.

*Edited on ***October 26, 2005, 2:23 pm**