This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A260407 #24 Sep 08 2022 08:46:13
%S A260407 1,17,257,8209,65537,649801,1382401,4294967297
%N A260407 Numbers n such that (n-1)^2+1 divides 2^(n-1)-1.
%C A260407 a(7) = 1382401 is the first composite number of this sequence (which makes it different from A260072).
%C A260407 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).
%C A260407 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
%F A260407 a(n) = A247165(n)+1.
%t A260407 Join [{1},Select[Range[43*10^8],PowerMod[2,#-1,(#-1)^2+1]==1&]] (* _Harvey P. Dale_, Sep 07 2018 *)
%o A260407 (PARI) forstep(n=1,1e7,2,Mod(2,(n-1)^2+1)^(n-1)==1&&print1(n","))
%o A260407 (Magma) [n: n in [1..10^6] | (2^(n-1)-1) mod ((n-1)^2+1) eq 0 ]; // _Vincenzo Librandi_, Jul 25 2015
%Y A260407 Cf. A000215, A081762, A247165, A260072, A260406.
%K A260407 nonn,more
%O A260407 1,2
%A A260407 _M. F. Hasler_, Jul 24 2015