OFFSET
1,2
LINKS
James Mitchell, Table of n, a(n) for n = 1..100
P. J. Cameron, M. Gadouleau, J. D. Mitchell, Y. Peresse, Chains of subsemigroups, arXiv preprint arXiv:1501.06394 [math.GR], 2015.
FORMULA
Conjecture: n*(131*n-376)*a(n) +2*(-563*n^2+1993*n-1185)*a(n-1) +3*(1099*n^2-4678*n+4684)*a(n-2) +2*(-1987*n^2+9803*n-12021)*a(n-3) +4*(209*n-387)*(2*n-7)*a(n-4)=0. - R. J. Mathar, Oct 20 2015
a(n) = binomial(2*n,n)/2 + 3*2^(n-1) - n - 2. - Gheorghe Coserea, May 16 2016
MATHEMATICA
a[n_] := Binomial[2n, n]/2 + 3*2^(n-1) - n - 2; Array[a, 30] (* Jean-François Alcover, Dec 15 2018, from PARI *)
PROG
(PARI) a(n)=-2-n+sum(i=0, n, binomial(n, i)*(binomial(n, i)+3)/2);
CROSSREFS
KEYWORD
nonn
AUTHOR
James Mitchell, May 14 2014
STATUS
approved