OFFSET
3,1
LINKS
F. Chapoton Cluster algebras
FORMULA
a(n) = (2*(n-1)/(2*n-1))*binomial(3*n-3,n-1)-binomial(3*n-5,n-2)+4*binomial(3*n-3,n-3).
a(n) = (16*n^2-41*n+24)/(n*(2*n-1))*binomial(3*n-5,n-2).
EXAMPLE
a(3)=12 because D3 is the same as A3.
PROG
(MuPAD)
modu_NC_D:=proc(n) begin (16*n*n-41*n+24)/n/(2*n-1)*binomial(3*n-5, n-2) end;
(Sage)
def A137207(n):
return (16*n*n-41*n+24)*binomial(3*n-5, n-2)/n/(2*n-1)
CROSSREFS
KEYWORD
nonn
AUTHOR
F. Chapoton, Mar 05 2008
EXTENSIONS
a(22)-a(24) from Stefano Spezia, Feb 29 2024
STATUS
approved