OFFSET
1,2
REFERENCES
E. M. Palmer and R. C. Read, On the number of plane 2-trees, J. London Math. Soc. (2), 6, 1973, 583-592.
FORMULA
a(n) = (5n-2)!/(4n-1)!.
E.g.f. T = T(x) satisfies T(1-T)^4 = x.
D-finite with recurrence -8*(4*n-3)*(2*n-1)*(4*n-1)*a(n) +5*(5*n-4)*(5*n-3)*(5*n-2)*(5*n-6)*a(n-1)=0. - R. J. Mathar, Jul 24 2022
MAPLE
seq((5*n-2)!/(4*n-1)!, n=1..16);
CROSSREFS
KEYWORD
nonn
AUTHOR
Emeric Deutsch, Jun 24 2006
STATUS
approved