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 A115411 a(n) = least k such that semiprime(n) divides k-th triangular number. 0
 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, 40, 34, 43, 29, 13, 30, 47, 19, 52, 36, 45, 59, 34, 120, 60, 41, 42, 56, 67, 47, 71, 65, 29, 72, 30, 79, 53, 69, 83, 168, 59, 88, 60, 74, 33, 96, 66, 100, 28, 40, 103, 76, 71, 107, 85 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS a(n) = MIN[k such that A001358(n) | A000217(k)]. LINKS EXAMPLE a(1) = 7 because SP(1) = semiprime(1) = 4, Triangular number T(7) = 7*(7+1)/2 = 28 and 7 divides 28. a(2) = 3 because SP(2) = 6 | T(3) = 6. a(3) = 8 because SP(3) = 9 | T(8) = 36. a(9) = 24 because SP(9) = 25 | T(24) = 300. a(17) = 48 because SP(17) = 49 | T(48) = 1176. MATHEMATICA 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 *) CROSSREFS Cf. A000217, A001358. Sequence in context: A111197 A245645 A335994 * A019726 A011330 A093587 Adjacent sequences:  A115408 A115409 A115410 * A115412 A115413 A115414 KEYWORD easy,nonn AUTHOR Jonathan Vos Post, Mar 08 2006 EXTENSIONS More terms from Robert G. Wilson v, May 01 2006 STATUS approved

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Last modified December 2 20:25 EST 2021. Contains 349445 sequences. (Running on oeis4.)