A135980 - OEIS

%I #12 May 09 2021 11:19:56

%S 5,9,10,12,13,14,15,16,17,19,20,21,22,23,25,26,27,29,30,32,33,34,35,

%T 36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,

%U 59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78

%N Numbers k such that the Mersenne number 2^prime(k)-1 is composite.

%C A135979 is a subsequence of this sequence.

%F prime(a(n)) = A054723(n).

%t k = {}; Do[If[ ! PrimeQ[2^Prime[n] - 1], AppendTo[k, n]], {n, 1, 40}]; k

%t m = PrimePi @ MersennePrimeExponent @ Range[13]; Complement[Range[m[[-1]]], m] (* _Amiram Eldar_, Mar 12 2020 *)

%o (PARI) isok(k) = !isprime(2^prime(k)-1); \\ _Michel Marcus_, Mar 12 2020

%Y Cf. A000225, A065341, A054723, A134852, A135975, A135976, A135977, A135978, A135979.

%K nonn

%O 1,1

%A _Artur Jasinski_, Dec 09 2007

%E More terms from _Amiram Eldar_, Mar 12 2020