a(n) = 2 * C(2*n,n-1).

Terms

a(0) =0a(1) =2a(2) =8a(3) =30a(4) =112a(5) =420a(6) =1584a(7) =6006a(8) =22880a(9) =87516a(10) =335920a(11) =1293292a(12) =4992288a(13) =19315400a(14) =74884320a(15) =290845350a(16) =1131445440a(17) =4407922860a(18) =17194993200a(19) =67156001220a(20) =262564816800