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 A057121 #3 May 01 2014 02:48:24 %S A057121 0,0,3,344,8398380,13286191841734681, %T A057121 89262894246121771416347364297566757 %N A057121 Local ranks of terms of A057119. %C A057121 CatalanRank given in A057117. %F A057121 a(n) = CatalanRank(2^n, bt_df2tree_apply_k_times(2, n)) %Y A057121 Cf. A057120. %K A057121 nonn %O A057121 0,3 %A A057121 _Antti Karttunen_, Aug 11 2000