Number of plane trees with even number of leaves.

Terms

a(0) =0a(1) =1a(2) =3a(3) =7a(4) =20a(5) =66a(6) =217a(7) =715a(8) =2424a(9) =8398a(10) =29414a(11) =104006a(12) =371384a(13) =1337220a(14) =4847637a(15) =17678835a(16) =64821680a(17) =238819350a(18) =883634026a(19) =3282060210a(20) =12233125112a(21) =45741281820a(22) =171529836218a(23) =644952073662