a(n) = floor(C(4n,2n)/C(4n,n)).

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =7a(5) =11a(6) =20a(7) =33a(8) =57a(9) =96a(10) =162a(11) =274a(12) =462a(13) =780a(14) =1317a(15) =2223a(16) =3751a(17) =6329a(18) =10680a(19) =18021a(20) =30409a(21) =51312a(22) =86584a(23) =146103a(24) =246538a(25) =416013a(26) =701993a(27) =1184566a(28) =1998882a(29) =3372999