A332549 Numbers k such that A332547(k) = 3.

5, 6, 8, 11, 12, 23, 47, 96, 191, 192, 383, 768, 6143, 12288, 786431, 786432, 3221225472, 51539607551, 206158430208, 824633720831, 6597069766656, 26388279066623, 108086391056891903, 55340232221128654847, 221360928884514619392, 226673591177742970257407

Offset: 1

Author

N. J. A. Sloane, Feb 21 2020

Keywords

• nonn

Comments

The numbers k such that A332547(k) = 1 are given by A068194, a sequence of interest to Mersenne and Fermat, so this sequence may also be interesting.

The factors of the initial terms are 5, 2*3, 2^3, 11, 2^2*3, 23, 47, 2^5*3, 191, 2^6*3, 383, 2^8*3, 6143, 2^12*3, 786431, 2^18*3, ...

There are essentially two cases. Firstly n can be an odd prime and n+1 of the form 3*2^k. These are the terms of A007505 with 2 excluded. Otherwise n can be of the form 3*2^k and n+1 a prime. These are 1 less than the terms of A039687. In addition, 8 is a term which is a special case. - Andrew Howroyd, Feb 21 2020

Crossrefs

Cf. A007505, A039687, A068194, A332547.

Programs

PARI

upto(n)={Set(concat([if(n<8,[],[8]), select(isprime, [3*2^k-1 |k<-[1..logint((n+1)\3, 2)]]), select(p->isprime(p+1), [3*2^k |k<-[1..logint(n\3, 2)]])]))} \\ Andrew Howroyd, Feb 21 2020

Extensions

Terms a(17) and beyond from Andrew Howroyd, Feb 21 2020