A Centered Polygonal Number consisting of a central dot with three dots around it, and then additional dots in the
gaps between adjacent dots. The general term is , and the first few such numbers are 1, 4, 10, 19, 31, 46,
64, ... (Sloane's A005448). The Generating Function giving the centered triangular numbers is
See also Centered Pentagonal Number, Centered Square Number
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.