A Figurate Number of the form , also called an Enneagonal Number. The first few are 1, 9, 24, 46, 75, 111, 154, 204, ... (Sloane's A001106).
Sloane, N. J. A. Sequence
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.