OFFSET
0,3
EXAMPLE
The triangle begins as follows (the slashes indicate the numbers b(0), b(1), b(2), ...):
1/
1 2
3 4 5
6 7 8 9/
1 2 3 4 5
6 7 8 9 10 11
12 13 14 15 16 17 18
19 20 21 22 23 24/ 1 2
3 4 ...
The initial terms in the rows give the sequence 1, 1, 3, 6, 1, 6, ...
MATHEMATICA
Module[{nn=20, p9}, p9=Flatten[Range[#]&/@PolygonalNumber[9, Range[nn]]]; TakeList[p9, Range[Floor[(Sqrt[1+8Length[p9]]-1)/2]]]][[All, 1]] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Jun 04 2020 *)
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Floor van Lamoen, Oct 22 2001
EXTENSIONS
Edited by N. J. A. Sloane, Jun 03 2020
STATUS
approved