a(n) = a(n-1) + (n-2)*a(n-2) + a(n-3) starting a(0)=0, a(1)=a(2)=1.

Terms

a(0) =0a(1) =1a(2) =1a(3) =2a(4) =5a(5) =12a(6) =34a(7) =99a(8) =315a(9) =1042a(10) =3661a(11) =13354a(12) =51006a(13) =201561a(14) =826987a(15) =3498286a(16) =15277665a(17) =68578942a(18) =316519868a(19) =1497639547a(20) =7263576113a(21) =36035247374a(22) =182804409181a(23) =946808180148