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 A027171 #7 Oct 24 2019 19:13:16 %S A027171 1,10,42,156,570,2096,7784,29168,110106,418128,1595616,6113740, %T A027171 23505352,90633796,350351636,1357278296,5268292826,20483876816, %U A027171 79765662896,311038321436,1214362277696,4746455801876,18570960418916 %N A027171 a(n) = A027170(2n, n). %K A027171 nonn %O A027171 0,2 %A A027171 _Clark Kimberling_