OFFSET
4,1
LINKS
Andrew Howroyd, Table of n, a(n) for n = 4..200
P. Flajolet and M. Noy, Analytic combinatorics of non-crossing configurations, Discrete Math., 204, 203-229, 1999.
FORMULA
a(n) = n*binomial(3n-3, n-4)/2.
D-finite with recurrence -2*(2*n+1)*(n-4)*a(n) +3*(3*n-4)*(3*n-5)*a(n-1)=0. - R. J. Mathar, Jul 26 2022
EXAMPLE
a(4)=2 because the only connected graphs on the nodes A,B,C,D having exactly two interior faces are {AB,BC,CD,DA,AC} and {AB,BC,CD,DA,BD}.
PROG
(PARI) a(n) = n*binomial(3*n-3, n-4)/2; \\ Michel Marcus, Oct 26 2015
CROSSREFS
KEYWORD
nonn
AUTHOR
Emeric Deutsch, Dec 28 2003
STATUS
approved