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 A138576 #61 Oct 22 2024 05:43:10 %S A138576 2,3,4,7,9,10,16,31,45,54,64,261,304,640,1102,1141,1609,2127,2212, %T A138576 4845,4971,5607,9969,10851,11605,22249,43122,55252,66025,108046, %U A138576 378420,429717,628894,699135,1488111,1510689,3486297,6733459,10498006,12018292,12982476,15201229,16291329,18578334 %N A138576 Numbers k such that 2^(2*k - 1) - 1 is prime. %H A138576 Mario Fernando Garcia Rivera, <a href="/A138576/b138576.txt">Table of n, a(n) for n = 1..47</a> (derived from A000043) %F A138576 a(n) = (A000043(n+1) + 1)/2. - _Charles R Greathouse IV_, Aug 30 2010 %F A138576 a(n) = A146768(n) + 1. - _César Aguilera_, May 27 2020 %e A138576 2^(2*2 - 1) - 1 = 7; %e A138576 2^(2*3 - 1) - 1 = 31; %e A138576 2^(2*4 - 1) - 1 = 127. %t A138576 (MersennePrimeExponent[Range[2, 48]] + 1)/2 (* _Amiram Eldar_, Oct 22 2024 *) %o A138576 (PARI) is(n)=isprime(2^(2*n-1)-1) \\ _Charles R Greathouse IV_, Jun 13 2017 %Y A138576 Cf. A000043, A146768. %K A138576 nonn %O A138576 1,1 %A A138576 _Vladimir Joseph Stephan Orlovsky_, May 13 2008 %E A138576 More terms from _Charles R Greathouse IV_, Aug 30 2010 %E A138576 More terms from _Mario Fernando Garcia Rivera_, Jul 18 2022