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 A048117 #4 Mar 30 2012 18:56:54 %S A048117 1,2,6,18,60,204,720,2580,9408,34668,129024,483672,1824768,6919752, %T A048117 26357976,100778472,386591304,1487221056,5735785176,22170562656, %U A048117 85866332760,333151068528,1294658562912,5038469947992,19634243736168 %N A048117 a(n)=(1/2)*T(2n,n), where T is given by A048113. %K A048117 nonn %O A048117 2,2 %A A048117 _Clark Kimberling_