a(n) is the number of rooted 3-connected bicubic planar maps with 2n vertices.

Terms

a(0) =1a(1) =0a(2) =0a(3) =1a(4) =0a(5) =3a(6) =7a(7) =15a(8) =63a(9) =168a(10) =561a(11) =1881a(12) =6110a(13) =21087a(14) =72174a(15) =250775a(16) =883116a(17) =3125910a(18) =11174280a(19) =40209852a(20) =145590720a(21) =530358095a(22) =1941862860a(23) =7144623447a(24) =26403493545a(25) =97971775008a(26) =364903633215