a(n) = C(4n,n-2).

Terms

a(0) =1a(1) =12a(2) =120a(3) =1140a(4) =10626a(5) =98280a(6) =906192a(7) =8347680a(8) =76904685a(9) =708930508a(10) =6540715896a(11) =60403728840a(12) =558383307300