Pentagonal Pyramidal Number

A Pyramidal Number of the form $n^2(n+1)/2$. The first few are 1, 6, 18, 40, 75, ... (Sloane's A002411). The Generating Function for the pentagonal pyramidal numbers is

$${x(2x+1)\over(x-1)^4}=x+6x^2+18x^3+40x^4+\ldots.$$

References

Sloane, N. J. A. Sequence A002411/M4116 in ``An On-Line Version of the Encyclopedia of Integer Sequences.'' http://www.research.att.com/~njas/sequences/eisonline.html and Sloane, N. J. A. and Plouffe, S. The Encyclopedia of Integer Sequences. San Diego: Academic Press, 1995.