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