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

Terms

a(0) =0a(1) =0a(2) =1a(3) =2a(4) =5a(5) =12a(6) =33a(7) =94a(8) =293a(9) =952a(10) =3297a(11) =11866a(12) =44837a(13) =175364a(14) =713409a(15) =2993142a(16) =12980869a(17) =57878000a(18) =265571905a(19) =1249497906a(20) =6029792197a(21) =29770252412a(22) =150366096353a(23) =775541397006