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%I #6 Mar 30 2012 17:22:57
%S 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,
%T 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,
%U 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
%N The index of the least n-gonal number greater than 1 that is also triangular, or 0 if none exists.
%F If n is a triangular number, then a(n)=2.
%t 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}]
%Y Cf. A188891.
%K nonn
%O 3,1
%A _T. D. Noe_, Apr 13 2011
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