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 A090612 #13 Jan 26 2018 03:58:50 %S A090612 2,8,21,38,153,191,232,279,327,378,493,559,1086,1272,1360,1769,1989, %T A090612 2111,2224,2344,2471,3272,3721,3863,4838,5006,5359,6291,6871,7077, %U A090612 7909,8127,9245,9928,10654,10889,12164,12957,13764,14881,16034,16343,16944 %N A090612 Numbers k such that the k-th prime is of the form 2*j^2 + 1. %C A090612 A090698 indexed by A000040. %H A090612 Robert Israel, <a href="/A090612/b090612.txt">Table of n, a(n) for n = 1..1525</a> %F A090612 a(n)=k such that A000040(k) = A090698(n) = 2*A089001(n)^2 + 1. %e A090612 From _Jon E. Schoenfield_, Jan 24 2018: (Start) %e A090612 prime(8) = 19 = 2*3^2 + 1, so 8 is in the sequence. %e A090612 prime(21) = 73 = 2*6^2 + 1, so 21 is in the sequence. %e A090612 prime(33) = 137 = 2*68 + 1, and 68 is not a square, so 33 is not in the sequence. (End) %p A090612 N:= 1000; # to get all entries corresponding to primes <= 2*N^2+1. %p A090612 R:= select(isprime,[seq(2*k^2+1,k=1..N)]): %p A090612 A090612:= map(numtheory[pi],R); # _Robert Israel_, May 09 2014 %t A090612 Select[Range[18000],IntegerQ[Sqrt[(Prime[#]-1)/2]]&] (* _Harvey P. Dale_, Apr 25 2016 *) %Y A090612 Cf. A089001, A089008, A090698. %K A090612 nonn %O A090612 1,1 %A A090612 _Ray Chandler_, Dec 21 2003