a(n) = 2^n*binomial(4*n, n)/(3*n+1).

Terms

a(0) =1a(1) =2a(2) =16a(3) =176a(4) =2240a(5) =31008a(6) =453376a(7) =6888960a(8) =107707392a(9) =1721477120a(10) =28000141312a(11) =461964898304