Number of labeled projective plane trees (or "flat" trees) with n nodes.

Terms

a(0) =1a(1) =1a(2) =1a(3) =2a(4) =3a(5) =6a(6) =12a(7) =27a(8) =65a(9) =175a(10) =490a(11) =1473a(12) =4588a(13) =14782a(14) =48678a(15) =163414a(16) =555885a(17) =1913334a(18) =6646728a(19) =23278989a(20) =82100014a(21) =291361744a(22) =1039758962a(23) =3729276257a(24) =13437206032a(25) =48620868106a(26) =176611864312a(27) =643834562075