A237251 Primes p such that p*2^(p-1)-1 is prime.

2, 3, 5, 17, 257, 16487

Offset

1,1

Comments

The fifth Fermat prime, 65537, is not in the sequence: 65537*2^65536-1 is composite (per PFGW). - Michael B. Porter, Feb 11 2014

Also 65537*2^65536-1 is divisible by 16267 and 2058772459. - Jeppe Stig Nielsen, Jan 04 2020

Links

Table of n, a(n) for n=1..6.

Prog

(PARI) isok(p) = isprime(p) && isprime(p*2^(p-1) - 1); \\ Michel Marcus, Feb 06 2014

Crossrefs

Cf. A019434, A092506, A230769, A236752.

Sequence in context: A347565 A265426 A099936 * A275584 A092506 A275159

Adjacent sequences: A237248 A237249 A237250 * A237252 A237253 A237254

Keyword

nonn,more,hard

Author

Gerasimov Sergey, Feb 05 2014

Extensions

a(5) from Ralf Stephan, Feb 03 2014

a(6) = A230769(26)+1 appended by Jeppe Stig Nielsen, Jan 04 2020

Status

approved