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

1,1

Links

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

Example

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

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 *)

Prog

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

Crossrefs

Cf. A066049.

Sequence in context: A340753 A239472 A234943 * A082910 A023646 A056225

Adjacent sequences: A209491 A209492 A209493 * A209495 A209496 A209497

Keyword

nonn

Author

Gerasimov Sergey, Mar 09 2012

Status

approved