a(n) = (n+1)*(a(n-1)/n + a(n-2)), with a(0)=1, a(1)=2.

Terms

a(0) =1a(1) =2a(2) =6a(3) =16a(4) =50a(5) =156a(6) =532a(7) =1856a(8) =6876a(9) =26200a(10) =104456a(11) =428352a(12) =1821976a(13) =7959056a(14) =35857200a(15) =165592576a(16) =785514512a(17) =3812387616a(18) =18948962656a(19) =96194028800a(20) =498931946016