a(n) = (2n)!/(phi(2n)!)^2.

Terms

a(0) =2a(1) =6a(2) =180a(3) =70a(4) =6300a(5) =831600a(6) =168168a(7) =12870a(8) =12350257920a(9) =1496523600a(10) =85357272a(12) =1757701400a(15) =601080390a(16) =674412197580