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 A067320 #9 Dec 15 2019 21:59:07 %S A067320 1,9,584,70576,10909440,1913027840,361936623616,72033971859456, %T A067320 14864066521333760,3151356047929704448,682330743447507959808, %U A067320 150243739893975187718144,33540494675674022306381824 %N A067320 Even-indexed members of A067297. %F A067320 a(n) = A067297(2n). %F A067320 G.f.: A(x) = (f(y) + f(-y))/2 with f(y) := (-1 + 9*y + (1+6*y)*c(y))/((1-y)*(1+5*y)*c(y) - 1 + 11*y - y^2) where c(y) is g.f. of A000108 (Catalan) and y=4*sqrt(x). %Y A067320 Cf. A067321 (one half of odd-indexed terms of A067297). %K A067320 nonn,easy %O A067320 0,2 %A A067320 _Wolfdieter Lang_, Feb 05 2002