A209494 Smallest prime p such that 2p*n^2 - 1 is prime, or 0 if no such prime exists.

2, 3, 3, 7, 3, 5, 3, 3, 5, 3, 7, 3, 3, 37, 13, 7, 3, 11, 31, 3, 5, 3, 19, 11, 13, 7, 3, 3, 19, 3, 31, 3, 5, 7, 3, 5, 13, 3, 11, 37, 61, 13, 3, 7, 3, 19, 73, 5, 7, 19, 11, 3, 31, 7, 3, 31, 31, 3, 7, 3, 19, 3, 3, 31, 3, 19, 151, 3, 5, 3, 7, 5, 3, 97

Offset: 1

Author

Gerasimov Sergey, Mar 09 2012

Keywords

• nonn

Examples

			2 is in the sequence because 2 and 2*2*1^2 - 1 = 3 are both primes.
		

Links

• Harvey P. Dale, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A066049.

Programs

Mathematica

spp[n_]:=Module[{n2=n^2,p=2},While[!PrimeQ[2p*n2-1],p=NextPrime[p]];p]; Array[spp,80] (* Harvey P. Dale, Jul 09 2017 *)

PARI

a(n)=forprime(p=2,default(primelimit),if(isprime(2*p*n^2-1),return(p))) \\ Charles R Greathouse IV, Mar 09 2012