A239742 Numbers k such that prime(k) * 2^k + 1 is prime.

1, 2, 3, 4, 5, 9, 12, 35, 78, 122, 276, 495, 715, 849, 927, 1128, 1263, 1418, 23201, 30316, 34894, 51844, 200821, 235383, 265970

Offset

1,2

Comments

The PFGW program has been used to certify all the terms up to a(25), using a deterministic test which exploits the factorization of a(n)-1.

a(26) > 325000.

Links

Table of n, a(n) for n=1..25.

Mathematica

Select[Range[1000], PrimeQ[Prime[#]* 2^# + 1] &]

Prog

(PARI) is(n, p=prime(n))=ispseudoprime(p<<n+1) \\ Charles R Greathouse IV, Jun 06 2017

Crossrefs

Cf. A239741.

Sequence in context: A038201 A033084 A076134 * A101526 A264994 A331364

Adjacent sequences: A239739 A239740 A239741 * A239743 A239744 A239745

Keyword

nonn,more

Author

Giovanni Resta, Mar 26 2014

Status

approved