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A051686 Smallest prime p such that 2n*p+1 is also prime. 9
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 (list; graph; refs; listen; history; text; internal format)
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 A076733

Adjacent sequences:  A051683 A051684 A051685 * A051687 A051688 A051689

KEYWORD

nonn

AUTHOR

Labos Elemer

STATUS

approved

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Last modified December 6 06:34 EST 2019. Contains 329784 sequences. (Running on oeis4.)