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

Terms

a(0) =0a(1) =1a(2) =1a(3) =3a(4) =8a(5) =20a(6) =66a(7) =196a(8) =688a(9) =2358a(10) =8840a(11) =33506a(12) =134376a(13) =550498a(14) =2350348a(15) =10273110a(16) =46413536a(17) =214598786a(18) =1020359628a(19) =4959234118a(20) =24699168280a(21) =125571468666a(22) =652484852756