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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 STATUS approved

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Last modified May 8 20:32 EDT 2021. Contains 343668 sequences. (Running on oeis4.)