a(n) = (n-2)*a(n-2) + a(n-3), with a(1)=0, a(2)=1, a(3)=1.

Terms

a(0) =0a(1) =1a(2) =1a(3) =2a(4) =4a(5) =9a(6) =22a(7) =58a(8) =163a(9) =486a(10) =1525a(11) =5023a(12) =17261a(13) =61801a(14) =229416a(15) =882475a(16) =3503041a(17) =14349016a(18) =60434172a(19) =261785329a(20) =1162598284a(21) =5296140752a(22) =24676349293a(23) =117677694828a(24) =572852174491