Centered 9-gonal (or nonagonal) numbers. Every third triangular number, starting with a(1)=1.
1, 10, 28, 55, 91, 136, 190, 253, 325, 406, 496, 595, 703, 820, 946, 1081, 1225, 1378
1 seqfan posts
Thu Dec 2 18:16:11 CET 2010 [seqfan] Re: Trotter Math