A276121 - OEIS

%I #14 Sep 28 2016 22:55:18

%S 1,1,1,1,3,1,1,1,13,7,1,21,15,3,31,147,45,1,43,73,15,69,91,1,51,81,3,

%T 1,9,85,1,55,169,225,109,145,15,103,615,69,259,69,63,45,285,471,9,255,

%U 169,489,69,273,427,43,391,169,201,21,159,181,103,15,339

%N Smallest odd integer k such that k*2^prime(n)-1 is a prime number.

%C When k=1 the prime k*2^prime(n)-1 is a Mersenne prime.

%H Pierre CAMI, <a href="/A276121/b276121.txt">Table of n, a(n) for n = 1..3000</a>

%F a(n) = A126717(prime(n)). - _Michel Marcus_, Sep 07 2016

%e 1*2^7-1 = 127 prime so a(4) = 1 as prime(4)=7.

%e 1*2^11-1 = 2047 composite, 3*2^11-1 = 6143 prime so a(5) = 3 as prime(5)=11.

%t Table[k = 1; While[! PrimeQ[k 2^Prime@ n - 1], k += 2]; k, {n, 63}] (* _Michael De Vlieger_, Aug 21 2016 *)

%o (PARI) a(n) = {my(k=1); while (!isprime(k*2^prime(n)-1), k+=2); k;} \\ _Michel Marcus_, Aug 21 2016

%Y Cf. A000043, A126717.

%K nonn

%O 1,5

%A _Pierre CAMI_, Aug 21 2016