2648967domain: NAppears in sequencesa(3)=1; a(n) = Sum_{i=1..n-3} binomial(n-4,i-1)*binomial(n,i+1)*a(i+2)*a(n-i)*i*(n-i-2)/(2*(n-1)) for n > 3.at n=6A115047Number of order-6 ribbon tilings for a 6 X n strip.at n=11A364426