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A188894
The index of the least n-gonal number greater than 1 that is also triangular, or 0 if none exists.
2
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
3,1
FORMULA
If n is a triangular number, then a(n)=2.
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}]
CROSSREFS
Cf. A188891.
Sequence in context: A293117 A293122 A014452 * A355416 A054579 A277962
KEYWORD
nonn
AUTHOR
T. D. Noe, Apr 13 2011
STATUS
approved