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 A152671 #8 Nov 03 2015 09:37:17 %S A152671 1,7,21,66,715,2431,8398,29393,104006,371450,1337220,17678835, %T A152671 64822395,238819350,883631595,3282060210,12233133510,45741281820, %U A152671 171529806825,644952073662,2430973200726,9183676536076,34766775458002 %N A152671 Even Catalan numbers divided by 2. %F A152671 a(n) = A152670(n)/2. %t A152671 Select[CatalanNumber[Range[30]], EvenQ]/2 (* _Vladimir Reshetnikov_, Nov 02 2015 *) %o A152671 (PARI) lista(nn) = for (n=1, nn, if (((c=binomial(2*n,n)/(n+1)) % 2) == 0, print1(c/2, ", "))); \\ _Michel Marcus_, Nov 02 2015 %Y A152671 Cf. A000108, A152670. %K A152671 easy,nonn %O A152671 1,2 %A A152671 _Omar E. Pol_, Dec 10 2008