a(n) = 4^n * binomial(3*n/2,n).

Terms

a(0) =1a(1) =6a(2) =48a(3) =420a(4) =3840a(5) =36036a(6) =344064a(7) =3325608a(8) =32440320a(9) =318704100a(10) =3148873728a(11) =31256180280a(12) =311452237824