Number of symmetric trivalent maps with n nodes.

Terms

a(0) =1a(1) =2a(2) =4a(3) =12a(4) =33a(5) =102a(6) =312a(7) =1010a(8) =3256a(9) =10836a(10) =36094a(11) =122544a(12) =417150a(13) =1437712a(14) =4970904a(15) =17333772a(16) =60638124a(17) =213435264a(18) =753520804a(19) =2672606464a(20) =9505230397a(21) =33928264990a(22) =121400935184a(23) =435660446342