Binomial coefficient C(4n,n-1).

Terms

a(0) =1a(1) =8a(2) =66a(3) =560a(4) =4845a(5) =42504a(6) =376740a(7) =3365856a(8) =30260340a(9) =273438880a(10) =2481256778a(11) =22595200368a(12) =206379406870