%I #60 Jun 27 2024 22:21:41
%S 1,18480,8388480
%N Numbers k such that 2*k^2 - 1 divides 2^k - 1.
%C a(4) > 2*10^10. - _Chai Wah Wu_, Dec 06 2014
%e 1 is in this sequence because 2*1^2 - 1 = 1 divides 2^1 - 1 = 1.
%o (Magma) [n: n in [1..100000] | Denominator((2^n-1)/(2*n^2-1)) eq 1];
%o (PARI) for(n=1,10^9,if(Mod(2,2*n^2-1)^n==+1,print1(n,", "))); \\ _Joerg Arndt_, Nov 30 2014
%Y Cf. A247219, A247221.
%K nonn,more,bref
%O 1,2
%A _Juri-Stepan Gerasimov_, Nov 30 2014
%E a(3) from _Joerg Arndt_, Nov 30 2014