A177464 The smallest positive k such that the n-th Mersenne prime +-k are two primes.

4, 12, 24, 30, 30, 66, 954, 1920, 30, 4116, 576, 214608

Offset: 2

Author

Vladimir Joseph Stephan Orlovsky, May 09 2010

Keywords

• nonn

Comments

Smallest k>0 such that A000668(n)+k and A000668(n)-k are both prime.

Examples

			7+-4->primes, 31+-12->primes, 127+-24->primes, 8191+-30->primes, 131071+-30->primes, 524287+-66->primes..
		

Crossrefs

Cf. A000668, A082467.

Programs

Mathematica

g[n_]:=2^Prime[n]-1; f[n_]:=Block[{k},If[OddQ[n],k=2,k=1];While[ !PrimeQ[n-k]||!PrimeQ[n+k],k+=2];k]; lst={};Do[If[PrimeQ[g[n]],AppendTo[lst,f[g[n]]]],{n,2,40}];lst

Formula

a(n) = A082467(A000668(n)). - R. J. Mathar, Jan 23 2011