A188894 The index of the least n-gonal number greater than 1 that is also triangular, or 0 if none exists.

2, 6, 12, 2, 5, 3, 10, 2, 0, 5, 3, 15, 2, 3, 6, 0, 11, 6, 2, 35, 3, 4, 6, 12, 3, 2, 10, 7, 8, 63, 4, 5, 285, 2, 20, 0, 6, 15, 3, 8, 9, 99, 2, 6, 18, 5, 15, 30, 6, 3, 21, 19, 2, 9, 10, 3, 6, 7, 5, 621, 14, 21, 42, 2, 55, 28, 4, 725, 3, 10, 11, 195, 7, 8, 460, 2, 14, 6, 28, 56, 10, 4, 36, 2107

Offset: 3

Author

T. D. Noe, Apr 13 2011

Keywords

• nonn

Crossrefs

Cf. A188891.

Programs

Mathematica

NgonIndex[n_, v_] := (-4 + n + Sqrt[16 - 8*n + n^2 - 16*v + 8*n*v])/(n - 2)/2; Table[k = 2; While[tr = k*(k+1)/2; i = NgonIndex[n, tr]; k < 20000 && ! IntegerQ[i], k++]; If[k==20000, i=0]; i, {n, 3, 50}]

Formula

If n is a triangular number, then a(n)=2.