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

Terms

a(0) =1a(1) =2a(2) =8a(3) =24a(4) =72a(5) =240a(6) =960a(7) =2880a(8) =15360a(9) =69120a(10) =165888a(11) =912384a(12) =3041280a(13) =9123840a(14) =34062336a(15) =127733760a(16) =479001600a(17) =1596672000a(18) =6386688000a(19) =22992076800a(20) =102187008000a(21) =429185433600