a(n) = a(n-1) + n * a(n-2), where a(1) = 1, a(2) = 2.

Terms

a(0) =1a(1) =2a(2) =5a(3) =13a(4) =38a(5) =116a(6) =382a(7) =1310a(8) =4748a(9) =17848a(10) =70076a(11) =284252a(12) =1195240a(13) =5174768a(14) =23103368a(15) =105899656a(16) =498656912a(17) =2404850720a(18) =11879332048a(19) =59976346448a(20) =309442319456