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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

Table of n, a(n) for n=1..15.

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.)