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%I #7 Jun 07 2013 12:31:39
%S 3,2,10,13,22,298,526,37,46,58,61,73,82,3397,2866,106,3481,3721,5293,
%T 5041,7081,157,166,178,193,10201,14317,23326,23761,226,17398,262,
%U 19042,277,24286,38806,313,45802,29893,346,358,32761,382,46126,52993,397,421,68461
%N a(n) = smallest index m such that smallest prime factor of m-th triangular number is prime(n).
%C Or, smallest proper divisor of m-th triangular number is prime(n).
%C The curve is bimodal. Why? - _T. D. Noe_, Jun 07 2013
%F A069901(a(n)) = A000040(n).
%e 3rd triangular number, A000217(3) = 6 = 2*3, 2nd triangular number, A000217(2) = 3,
%e 10th triangular number, A000217(10) = 55 = 5*11, 13th triangular number, A000217(13) = 91 =7*13.
%t nn = 50; t = Table[0, {nn}]; tri = 1; n = 1; found = 0; While[found < nn, n++; tri = tri + n; p = FactorInteger[tri][[1, 1]]; pi = PrimePi[p]; If[pi <= nn && t[[pi]] == 0, t[[pi]] = n; found++]]; t (* _T. D. Noe_, Jun 07 2013 *)
%Y Cf. A000040, A000217, A069901.
%K nonn
%O 1,1
%A _Zak Seidov_, Jun 06 2013
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