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 A125768 Least number k such that n*T_k +/- 1 is prime. 6
 1, 1, 1, 1, 3, 1, 3, 1, 3, 1, 3, 1, 3, 1, 3, 1, 3, 1, 3, 1, 3, 1, 3, 1, 3, 2, 3, 1, 3, 1, 4, 1, 3, 2, 3, 1, 3, 1, 3, 1, 4, 1, 3, 1, 3, 1, 3, 1, 3, 2, 3, 1, 3, 1, 3, 2, 4, 1, 3, 1, 3, 1, 3, 2, 3, 1, 3, 1, 4, 1, 4, 1, 3, 1, 3, 2, 3, 1, 7, 1, 3, 1, 3, 1, 3, 2, 3, 1, 8, 1, 3, 2, 3, 2, 3, 1, 4, 1, 3, 1, 3, 1, 3, 1, 3 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,5 COMMENTS k times the n-th triangular number plus or minus 1 is prime. Eventually every prime p appears since (p +/-1) times 1(1+1)/2 equals (p +/- 1). The first occurrence of k: 1, 26, 5, 31, 320, 698, 79, 89, 1058, 7404, 223, 299, 8504, 13630, 845, 1217, 3334, 26074, 2349, 1835, 71926, 111616, 2473, 1189, 27754, 140366, 3709, 4109, 250126, 223078, 30359, 9649, ...,. LINKS MATHEMATICA f[n_] := Block[{k = 1, trnglr = n*k(k + 1)/2}, While[ !PrimeQ[trnglr - 1] && !PrimeQ[trnglr + 1], k++ ]; k]; Array[f, 105] CROSSREFS Cf. A000217, A125765, A125766, A125767, A125769. Sequence in context: A112030 A010684 A176040 * A266875 A307193 A111742 Adjacent sequences:  A125765 A125766 A125767 * A125769 A125770 A125771 KEYWORD nonn AUTHOR Jonathan Vos Post & Robert G. Wilson v, Dec 01 2006 STATUS approved

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Last modified April 3 19:43 EDT 2020. Contains 333198 sequences. (Running on oeis4.)