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

Terms

a(0) =1a(1) =2a(2) =3a(3) =7a(4) =16a(5) =44a(6) =124a(7) =388a(8) =1256a(9) =4360a(10) =15664a(11) =59264a(12) =231568a(13) =942736a(14) =3953120a(15) =17151424a(16) =76448224a(17) =350871008a(18) =1650490816a(19) =7966168960a(20) =39325494464a(21) =198648873664