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 A223701 Irregular triangle of numbers k such that prime(n) is the largest prime factor of k^2 - 1. 7
 3, 2, 5, 7, 17, 4, 9, 11, 19, 26, 31, 49, 161, 6, 8, 13, 15, 29, 41, 55, 71, 97, 99, 127, 244, 251, 449, 4801, 8749, 10, 21, 23, 34, 43, 65, 76, 89, 109, 111, 197, 199, 241, 351, 485, 769, 881, 1079, 6049, 19601, 12, 14, 25, 27, 51, 53, 64, 79, 129, 131, 155 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Note that the first number of each row forms the sequence 3, 2, 4, 6, 10, 12,..., which is A039915. The rows, except the first, are in A181447-A181470. LINKS EXAMPLE Irregular triangle: {3}, {2, 5, 7, 17}, {4, 9, 11, 19, 26, 31, 49, 161}, {6, 8, 13, 15, 29, 41, 55, 71, 97, 99, 127, 244, 251, 449, 4801, 8749} MATHEMATICA t = Table[FactorInteger[n^2 - 1][[-1, 1]], {n, 2, 10^5}]; Table[1 + Flatten[Position[t, Prime[n]]], {n, 6}] CROSSREFS Cf. A175607 (largest number k such that the greatest prime factor of k^2-1 is prime(n)). Cf. A181447-A181470. Cf. A223702-A223707 (related sequences). Sequence in context: A171018 A239260 A013655 * A220519 A094894 A089334 Adjacent sequences:  A223698 A223699 A223700 * A223702 A223703 A223704 KEYWORD nonn,tabf AUTHOR T. D. Noe, Apr 03 2013 STATUS approved

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Last modified August 12 21:10 EDT 2020. Contains 336440 sequences. (Running on oeis4.)