a(n) = a(n-1) + n*a(n-2); a(0) = a(1) = 1.

Terms

a(0) =1a(1) =1a(2) =3a(3) =6a(4) =18a(5) =48a(6) =156a(7) =492a(8) =1740a(9) =6168a(10) =23568a(11) =91416a(12) =374232a(13) =1562640a(14) =6801888a(15) =30241488a(16) =139071696a(17) =653176992a(18) =3156467520a(19) =15566830368a(20) =78696180768a(21) =405599618496