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 A027135 #8 Oct 23 2019 21:57:01
%S A027135 1,3,11,46,180,707,2729,10377,39045,145493,537749,1973915,7202499,
%T A027135 26144901,94475783,340028026,1219451204,4359473611,15540448583,
%U A027135 55255188413,196004642045,693798921723,2451058691129,8643625483966
%N A027135 a(n) = Sum_{k=0..n} T(n,k) * T(n,2n-k), with T given by A027113.
%K A027135 nonn
%O A027135 0,2
%A A027135 _Clark Kimberling_