a(n) = A081470(n)/n.

Terms

a(0) =2a(1) =6a(2) =40a(3) =420a(4) =6048a(5) =84a(6) =2471040a(7) =1485a(8) =400400a(9) =36036a(11) =3640a(13) =41860800a(14) =124032a(15) =3357585a(17) =5383840a(19) =318780a(20) =284169600