A051686 Smallest prime p such that 2n*p+1 is also prime.

2, 3, 2, 2, 3, 3, 2, 7, 2, 2, 3, 3, 2, 7, 2, 3, 3, 2, 5, 7, 3, 2, 3, 2, 2, 3, 2, 2, 19, 3, 5, 3, 3, 2, 3, 13, 2, 3, 2, 3, 19, 5, 2, 7, 2, 3, 3, 2, 2, 7, 3, 3, 7, 5, 3, 3, 2, 2, 7, 2, 3, 3, 3, 2, 7, 3, 2, 3, 2, 2, 13, 3, 2, 37, 5, 3, 3, 2, 2, 13, 3, 5, 3, 2, 11, 13, 2, 2, 31, 3, 3, 7, 2, 5, 3, 3, 2, 7, 2, 2

Offset

1,1

Comments

These are the primes arising in A051899.

Links

Michael De Vlieger, Table of n, a(n) for n = 1..10000

Example

a(29)=19 because 19 is the smallest prime p such that 2*29*p+1 is prime.

Mathematica

Table[k = 1; While[! PrimeQ[2 n Prime@ k + 1], k++]; Prime@ k, {n, 120}] (* Michael De Vlieger, Jul 26 2016 *)

Prog

(PARI) a(n) = p=2; while(!isprime(2*n*p+1), p = nextprime(p+1)); p; \\ Michel Marcus, Dec 10 2013

Crossrefs

Cf. A005384, A051644-A051654, A051899.

Sequence in context: A240755 A309806 A256170 * A079294 A242789 A366712

Adjacent sequences: A051683 A051684 A051685 * A051687 A051688 A051689

Keyword

nonn

Author

Labos Elemer

Status

approved