Number of closable Motzkin trees.

Terms

a(0) =0a(1) =1a(2) =1a(3) =2a(4) =5a(5) =11a(6) =26a(7) =65a(8) =163a(9) =417a(10) =1086a(11) =2858a(12) =7599a(13) =20391a(14) =55127a(15) =150028a(16) =410719a(17) =1130245a(18) =3124770a(19) =8675210a(20) =24175809a(21) =67603633a(22) =189633981a(23) =533463183a(24) =1504644945a(25) =4254179693a(26) =12055097308a(27) =34231674486a(28) =97392368007a(29) =277590288931