OFFSET
0,3
COMMENTS
Also number of even-length branches starting at the root in all ordered trees with n+1 edges. - Emeric Deutsch, Mar 03 2007
Also number of Dyck paths of semi-length n+1 with first descent and last ascent of equal size.
LINKS
G. C. Greubel, Table of n, a(n) for n = 0..1000 (terms 0..200 from Vincenzo Librandi)
FORMULA
a(n) = Sum_{k=0..n} k*A127538(n,k).
G.f.: x*C/(1-x^2-x*C-x^2*C), where C = (1-sqrt(1-4*x))/(2*x) is the Catalan function.
a(n) ~ 3*4^(n+1)/(5*sqrt(Pi)*n^(3/2)). - Vaclav Kotesovec, Mar 21 2014
D-finite with recurrence (n+2)*a(n) -4*n*a(n-1) +(-n-8)*a(n-2) +2*(2*n-3)*a(n-3)=0. - R. J. Mathar, Jul 26 2022
EXAMPLE
a(2)=2 because the tree /\ has two odd-length branches starting from the root and the path-tree of length 2 has none.
a(2)=2 because the Dyck paths of semi-length 3 with first descent and last ascent of same size are UUDUDD and UDUDUD.
MAPLE
C:=(1-sqrt(1-4*z))/2/z: g:=z*C/(1-z^2-z*C-z^2*C): gser:=series(g, z=0, 32): seq(coeff(gser, z, n), n=0..29);
MATHEMATICA
CoefficientList[Series[x (1 - (1 - 4*x)^(1/2))/(2*x)/(1 - x^2 - x *(1 - (1 - 4*x)^(1/2)) /(2*x) - x^2*(1 - (1 - 4*x)^(1/2))/(2*x)), {x, 0, 40}], x] (* Vaclav Kotesovec, Mar 21 2014 *)
PROG
(PARI) concat([0], Vec(x*(1 - (1 - 4*x)^(1/2))/(2*x)/(1 - x^2 - x*(1 - (1 - 4*x)^(1/2)) /(2*x) - x^2*(1 - (1 - 4*x)^(1/2))/(2*x)) + O(x^50))) \\ G. C. Greubel, Jan 31 2017
CROSSREFS
KEYWORD
nonn
AUTHOR
Emeric Deutsch, Mar 01 2007
STATUS
approved