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

Terms

a(0) =0a(1) =1a(2) =4a(3) =16a(4) =96a(5) =288a(6) =1440a(7) =5760a(8) =23040a(9) =92160a(10) =552960a(11) =1658880a(12) =7299072a(13) =36495360a(14) =109486080a(15) =510935040a(16) =2043740160a(17) =7664025600a(18) =28740096000a(19) =114960384000a(20) =459841536000