Number of 3-valent trees (= boron trees or binary trees) with n nodes.

Terms

a(0) =1a(1) =1a(2) =0a(3) =1a(4) =0a(5) =1a(6) =0a(7) =1a(8) =0a(9) =2a(10) =0a(11) =2a(12) =0a(13) =4a(14) =0a(15) =6a(16) =0a(17) =11a(18) =0a(19) =18a(20) =0a(21) =37a(22) =0a(23) =66a(24) =0a(25) =135a(26) =0a(27) =265a(28) =0a(29) =552