A067320 - OEIS

%I #9 Dec 15 2019 21:59:07

%S 1,9,584,70576,10909440,1913027840,361936623616,72033971859456,

%T 14864066521333760,3151356047929704448,682330743447507959808,

%U 150243739893975187718144,33540494675674022306381824

%N Even-indexed members of A067297.

%F a(n) = A067297(2n).

%F 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 Cf. A067321 (one half of odd-indexed terms of A067297).

%K nonn,easy

%O 0,2

%A _Wolfdieter Lang_, Feb 05 2002