A247132 Numbers k such that 2k - 1 divides 2^k + 1.
1, 194997, 1463649, 1957025, 4657005, 6464145, 17214725, 70930629, 76938345, 319359365, 336837501, 429872625, 486213525, 1343289717, 1831683645, 2163016845, 2430979425, 2950546137, 3463374005, 5031564525, 5608791441, 8993704797, 9596401485, 12556945401, 13492461125, 14559291285, 18429009725
Offset: 1
Keywords
Examples
1 is in this sequence because (2^1 + 1)/(2*1 - 1) = 3.
Programs
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Magma
[n: n in [1..2000000] | Denominator((2^n + 1)/(2*n - 1)) eq 1];
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Mathematica
Select[Range[200000], IntegerQ[(2^# + 1) / (2 # - 1)] &] (* Vincenzo Librandi, Nov 20 2014 *)
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PARI
is(n)=Mod(2,2*n-1)^n==-1 \\ Charles R Greathouse IV, Nov 20 2014
Extensions
a(5)-a(27) from Charles R Greathouse IV, Nov 20 2014