a(n) = n*a(n-1) + (n-4)*a(n-2), a(2) = 0, a(3) = 1.
A001909
a(n) = n*a(n-1) + (n-4)*a(n-2), a(2) = 0, a(3) = 1.
Terms
- a(0) =0a(1) =1a(2) =4a(3) =21a(4) =134a(5) =1001a(6) =8544a(7) =81901a(8) =870274a(9) =10146321a(10) =128718044a(11) =1764651461a(12) =25992300894a(13) =409295679481
External references
- oeis: A001909