A247132 - OEIS

%I #21 Jun 27 2024 22:21:37

%S 1,194997,1463649,1957025,4657005,6464145,17214725,70930629,76938345,

%T 319359365,336837501,429872625,486213525,1343289717,1831683645,

%U 2163016845,2430979425,2950546137,3463374005,5031564525,5608791441,8993704797,9596401485,12556945401,13492461125,14559291285,18429009725

%N Numbers k such that 2k - 1 divides 2^k + 1.

%e 1 is in this sequence because (2^1 + 1)/(2*1 - 1) = 3.

%t Select[Range[200000], IntegerQ[(2^# + 1) / (2 # - 1)] &] (* _Vincenzo Librandi_, Nov 20 2014 *)

%o (Magma) [n: n in [1..2000000] | Denominator((2^n + 1)/(2*n - 1)) eq 1];

%o (PARI) is(n)=Mod(2,2*n-1)^n==-1 \\ _Charles R Greathouse IV_, Nov 20 2014

%Y Cf. A081856, A081858, A247094.

%K nonn

%O 1,2

%A _Juri-Stepan Gerasimov_, Nov 20 2014

%E a(5)-a(27) from _Charles R Greathouse IV_, Nov 20 2014