a(n) = binomial(2n+1,n-3).

Terms

a(0) =1a(1) =9a(2) =55a(3) =286a(4) =1365a(5) =6188a(6) =27132a(7) =116280a(8) =490314a(9) =2042975a(10) =8436285a(11) =34597290a(12) =141120525a(13) =573166440a(14) =2319959400a(15) =9364199760a(16) =37711260990a(17) =151584480450a(18) =608359048206