Number of unlabeled 4-gonal 2-trees with n 4-gons.

Terms

a(0) =1a(1) =1a(2) =1a(3) =3a(4) =8a(5) =32a(6) =141a(7) =749a(8) =4304a(9) =26492a(10) =169263a(11) =1115015a(12) =7507211a(13) =51466500a(14) =358100288a(15) =2523472751a(16) =17978488711a(17) =129325796854a(18) =938234533024