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 A048121 #6 Jul 28 2022 03:52:27 %S A048121 0,0,1,3,12,42,156,570,2124,7926,29844,112812,428712,1635108,6259236, %T A048121 24031260,92517180,357019224,1380671532,5349496152,20762603724, %U A048121 80709201504,314178212928,1224576411084,4778651920092,18667769062044 %N A048121 a(n) = (1/2)*T(2n,n+1), where T is given by A048113. %Y A048121 Cf. A048113. %K A048121 nonn %O A048121 1,4 %A A048121 _Clark Kimberling_