a(n) = floor(C(2n,n)/2^(n+1)).

Terms

a(0) =0a(1) =0a(2) =0a(3) =1a(4) =2a(5) =3a(6) =7a(7) =13a(8) =25a(9) =47a(10) =90a(11) =172a(12) =330a(13) =634a(14) =1224a(15) =2366a(16) =4585a(17) =8902a(18) =17309a(19) =33707a(20) =65730a(21) =128330a(22) =250828a(23) =490750a(24) =961053a(25) =1883664a(26) =3694880a(27) =7252914a(28) =14246796a(29) =28002324