This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A209494 #14 Jul 09 2017 16:20:31
%S A209494 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,
%T A209494 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,
%U A209494 7,3,19,3,3,31,3,19,151,3,5,3,7,5,3,97
%N A209494 Smallest prime p such that 2p*n^2 - 1 is prime, or 0 if no such prime exists.
%H A209494 Harvey P. Dale, <a href="/A209494/b209494.txt">Table of n, a(n) for n = 1..1000</a>
%e A209494 2 is in the sequence because 2 and 2*2*1^2 - 1 = 3 are both primes.
%t A209494 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 *)
%o A209494 (PARI) a(n)=forprime(p=2,default(primelimit),if(isprime(2*p*n^2-1),return(p))) \\ _Charles R Greathouse IV_, Mar 09 2012
%Y A209494 Cf. A066049.
%K A209494 nonn
%O A209494 1,1
%A A209494 _Gerasimov Sergey_, Mar 09 2012