Number of planar 2-trees with n nodes.

Terms

a(0) =1a(1) =2a(2) =10a(3) =83a(4) =690a(5) =6412a(6) =61842a(7) =617025a(8) =6302725a(9) =65619820a(10) =693643860a(11) =7425181257a(12) =80326705270a(13) =876832043840