OFFSET
1,2
COMMENTS
a(7) = 1382401 is the first composite number of this sequence (which makes it different from A260072).
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 A260406 (except for the conventional 1).
Conjecture: also numbers n such that ((2^k)^(n-1)-1) == 0 mod ((n-1)^2+1) for all k >= 1. - Jaroslav Krizek, Jun 02 2016
FORMULA
a(n) = A247165(n)+1.
MATHEMATICA
Join [{1}, Select[Range[43*10^8], PowerMod[2, #-1, (#-1)^2+1]==1&]] (* Harvey P. Dale, Sep 07 2018 *)
PROG
(PARI) forstep(n=1, 1e7, 2, Mod(2, (n-1)^2+1)^(n-1)==1&&print1(n", "))
(Magma) [n: n in [1..10^6] | (2^(n-1)-1) mod ((n-1)^2+1) eq 0 ]; // Vincenzo Librandi, Jul 25 2015
CROSSREFS
KEYWORD
nonn,more
AUTHOR
M. F. Hasler, Jul 24 2015
STATUS
approved