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 A273870 Numbers n such that 4^(n-1) == 1 (mod (n-1)^2+1). 2
 1, 3, 5, 17, 217, 257, 387, 8209, 20137, 37025, 59141, 65537, 283801, 649801, 1382401 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also, numbers n such that (4^k)^(n-1) == 1 (mod (n-1)^2+1) for all k >= 0. Contains Fermat numbers (A000215) as subsequence. Prime terms are in A273871. LINKS EXAMPLE 5 is term because 4^(5-1) == 1 (mod (5-1)^2+1), i.e., 255 == 0 (mod 17). PROG (MAGMA) [n: n in [1..100000] | (4^(n-1)-1) mod ((n-1)^2+1) eq 0] (PARI) isok(n) = Mod(4, (n-1)^2+1)^(n-1) == 1; \\ Michel Marcus, Jun 02 2016 CROSSREFS Cf. A000215, A019434, A273871. Sequence in context: A171271 A056826 A278138 * A272060 A058910 A023394 Adjacent sequences:  A273867 A273868 A273869 * A273871 A273872 A273873 KEYWORD nonn,more AUTHOR Jaroslav Krizek, Jun 01 2016 EXTENSIONS a(14)-a(15) from Michel Marcus, Jun 02 2016 Edited by Max Alekseyev, Apr 30 2018 STATUS approved

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Last modified April 23 02:15 EDT 2019. Contains 322380 sequences. (Running on oeis4.)