OFFSET
0,2
LINKS
F. Disanto and S. Rinaldi, Symmetric convex permutominoes and involutions, PU. M. A., Vol. 22 (2011), No. 1, pp. 39-60; See M_n.
FORMULA
G.f.: (3-4*x-sqrt(1-4*x^2))/(2*(1-2*x)^2).
D-finite with recurrence: n*(n-4)*a(n) +2*(5+2n-n^2)*a(n-1) -4*(n-2)*(n-5)*a(n-2) +8*(n-2)*(n-3)*a(n-3)=0. - R. J. Mathar, Jun 28 2012
a(n) ~ n*2^(n-1) * (1-2*sqrt(2/Pi)/sqrt(n)). - Vaclav Kotesovec, Jun 29 2013
CROSSREFS
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, May 05 2012
STATUS
approved