Search: id:A115411
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%I A115411
%S A115411 7,3,8,4,7,5,6,11,24,12,11,16,14,19,12,23,48,17,10,18,28,31,25,23,36,21,
%T A115411 40,34,43,29,13,30,47,19,52,36,45,59,34,120,60,41,42,56,67,47,71,65,29,
%U A115411 72,30,79,53,69,83,168,59,88,60,74,33,96,66,100,28,40,103,76,71,107,85
%N A115411 a(n) = least k such that semiprime(n) divides k-th triangular number.
%C A115411 a(n) = MIN[k such that A001358(n) | A000217(k)].
%e A115411 a(1) = 7 because SP(1) = semiprime(1) = 4, Triangular number T(7) = 7*(7+1)/
               2 = 28 and 7 divides 28.
%e A115411 a(2) = 3 because SP(2) = 6 | T(3) = 6.
%e A115411 a(3) = 8 because SP(3) = 9 | T(8) = 36.
%e A115411 a(9) = 24 because SP(9) = 25 | T(24) = 300.
%e A115411 a(17) = 48 because SP(17) = 49 | T(48) = 1176.
%t A115411 a = Select[Range@215, Plus @@ Last /@ FactorInteger@# == 2 &]; f[n_] 
               := Block[{k = 1}, While[ Mod[k(k + 1)/2, a[[n]]] > 0, k++ ]; k]; 
               Array[f, 71] (* Robert G. Wilson v *)
%Y A115411 Cf. A000217, A001358.
%Y A115411 Sequence in context: A160578 A166201 A111197 this_sequence A019726 A011330 
               A093587
%Y A115411 Adjacent sequences: A115408 A115409 A115410 this_sequence A115412 A115413 
               A115414
%K A115411 easy,nonn
%O A115411 1,1
%A A115411 Jonathan Vos Post (jvospost3(AT)gmail.com), Mar 08 2006
%E A115411 More terms from Robert G. Wilson v (rgwv(at)rgwv.com), May 01 2006

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