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 A171820 #10 Feb 07 2017 14:30:04 %S A171820 2,4,6,8,11,12,14,18,19,21,22,25,27,29,31,34,36,37,38,42,44,46,47,48, %T A171820 50,53,58,59,61,63,65,67,68,70,73,74,75,78,80,82,84,85,88,90,93,95,99, %U A171820 100,101,105,106,110,111,112,114,115,117,121,122,125,127,129,130,131,133 %N A171820 Numbers n such that the n-th prime is of the form 3k + 1/2 +- 1/2. %C A171820 Numbers n such that the n-th prime is a generalized Cuban prime (A007645). A171820 UNION A091177 = A000027. %C A171820 Essentially the same as A091178. - _R. J. Mathar_, Jan 28 2010 %e A171820 2 is in the sequence because 2nd prime = 3*1 + 1/2 - 1/2; %e A171820 4 is in the sequence because 4th prime = 3*2 + 1/2 + 1/2. %o A171820 (PARI) list(lim)=my(v=List(),n); forprime(p=2,, if(n++>lim, return(Vec(v))); if(p%3<2, listput(v,n))) \\ _Charles R Greathouse IV_, Feb 07 2017 %Y A171820 Cf. A000027, A000040, A007645, A091177. %K A171820 nonn,less %O A171820 1,1 %A A171820 _Juri-Stepan Gerasimov_, Dec 19 2009