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 A135980 #15 Apr 08 2025 23:29:07
%S A135980 5,9,10,12,13,14,15,16,17,19,20,21,22,23,25,26,27,29,30,32,33,34,35,
%T A135980 36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,
%U A135980 59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78
%N A135980 Numbers k such that the Mersenne number 2^prime(k)-1 is composite.
%C A135980 A135979 is a subsequence of this sequence.
%F A135980 prime(a(n)) = A054723(n).
%F A135980 a(n) = pi(A054723(n)).
%t A135980 k = {}; Do[If[ ! PrimeQ[2^Prime[n] - 1], AppendTo[k, n]], {n, 1, 40}]; k
%t A135980 m = PrimePi @ MersennePrimeExponent @ Range[13]; Complement[Range[m[[-1]]], m] (* _Amiram Eldar_, Mar 12 2020 *)
%o A135980 (PARI) isok(k) = !isprime(2^prime(k)-1); \\ _Michel Marcus_, Mar 12 2020
%Y A135980 Cf. A000225, A065341, A054723, A134852, A135975, A135976, A135977, A135978, A135979.
%K A135980 nonn
%O A135980 1,1
%A A135980 _Artur Jasinski_, Dec 09 2007
%E A135980 More terms from _Amiram Eldar_, Mar 12 2020