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 A125603 #8 Mar 09 2017 17:03:59 %S A125603 4,5,9,12,17,20,21,24,32,37,40,45,49,56,57,69,72,77,81,84,85,96,100, %T A125603 104,105,109,116,117,125,132,136,140,141,145,152,157,164,165,169,172, %U A125603 181,185,189,192,196,204,205,216,217,220,221,224,245,257,264,269,272,277 %N A125603 Numbers n such that 3n(n-1)/2 + 1 is prime. %H A125603 Harvey P. Dale, <a href="/A125603/b125603.txt">Table of n, a(n) for n = 1..1000</a> %t A125603 Select[Range[300],PrimeQ[3# (#-1)/2+1]&] (* _Harvey P. Dale_, Mar 09 2017 *) %o A125603 (PARI) isok(n) = isprime(3*n*(n-1)/2 + 1); \\ _Michel Marcus_, Oct 11 2013 %Y A125603 Cf. A005448, A125602. %K A125603 nonn %O A125603 1,1 %A A125603 _Zak Seidov_, Nov 27 2006