a(n) = phi(binomial(2*n,n)*n^2).

Terms

a(0) =0a(1) =1a(2) =8a(3) =48a(4) =384a(5) =1440a(6) =8640a(7) =40320a(8) =184320a(9) =829440a(10) =5529600a(11) =18247680a(12) =87588864a(13) =474439680a(14) =1532805120a(15) =7664025600a(16) =32699842560a(17) =130288435200a(18) =517321728000