a(n) = [ C(2n,n)/(n-1) ].

Terms

a(0) =6a(1) =10a(2) =23a(3) =63a(4) =184a(5) =572a(6) =1838a(7) =6077a(8) =20528a(9) =70543a(10) =245832a(11) =866716a(12) =3085892a(13) =11079822a(14) =40072026a(15) =145850388a(16) =533831488a(17) =1963625766a(18) =7255080464a(19) =26912893722a(20) =100195188748a(21) =374246851254