a(0) = -1, a(1) = 1, a(n) = a(n-1) + (n-1)*a(n-2) for n > 1.

Terms

a(0) =-1a(1) =1a(2) =0a(3) =2a(4) =2a(5) =10a(6) =20a(7) =80a(8) =220a(9) =860a(10) =2840a(11) =11440a(12) =42680a(13) =179960a(14) =734800a(15) =3254240a(16) =14276240a(17) =66344080a(18) =309040160a(19) =1503233600a(20) =7374996640a(21) =37439668640a(22) =192314598080