A067320 Even-indexed members of A067297.

1, 9, 584, 70576, 10909440, 1913027840, 361936623616, 72033971859456, 14864066521333760, 3151356047929704448, 682330743447507959808, 150243739893975187718144, 33540494675674022306381824

Offset: 0

Author

Wolfdieter Lang, Feb 05 2002

Keywords

• nonn
• easy

Crossrefs

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

Formula

a(n) = A067297(2n).

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).