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 A260406 Numbers n such that (n-1)^2-1 divides 2^(n-1)-1. 3
 1, 3, 5, 17, 37, 257, 457, 1297, 2557, 4357, 6481, 8009, 11953, 26321, 44101, 47521, 47881, 49681, 57241, 65537, 74449, 84421, 97813, 141157, 157081, 165601, 225457, 278497, 310591, 333433, 365941, 403901, 419711, 476737, 557041, 560737, 576721, 647089, 1011961 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS The initial 1 is conventional. 647089 is the smallest composite number of this sequence (which makes it different from A081762). The next composite number in this sequence is a(1000) = F_5 = 4294967297. - Robert G. Wilson v, Jul 25 2015 The Fermat numbers 2^2^k+1 = A000215(k) with k>1 are a subsequence of this sequence. I conjecture that they are equal to the intersection of this and A260407 (apart from the conventional 1), i.e., the numbers such that (n-1)^4-1 divides 2^(n-1)-1. LINKS Robert G. Wilson v, Table of n, a(n) for n = 1..1598 MATHEMATICA fQ[n_] := PowerMod[2, n - 1, (n - 1)^2 - 1] == 1; Select[ Range[3, 1200000], fQ] (* Robert G. Wilson v, Jul 25 2015 *) PROG (PARI) forstep(n=1, 1e7, 2, Mod(2, (n-1)^2-1)^(n-1)==1&&print1(n", ")) (MAGMA) [n: n in [3..6*10^5] | (2^(n-1)-1) mod ((n-1)^2-1) eq 0]; // Vincenzo Librandi, Jul 26 2015 CROSSREFS Cf. A081762, A260072, A260407. Sequence in context: A305411 A019414 A249131 * A081762 A148515 A148516 Adjacent sequences:  A260403 A260404 A260405 * A260407 A260408 A260409 KEYWORD nonn AUTHOR M. F. Hasler, Jul 24 2015 STATUS approved

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Last modified February 19 00:54 EST 2020. Contains 332028 sequences. (Running on oeis4.)