Number of rooted planar 2-trees with n nodes.

Terms

a(0) =1a(1) =2a(2) =4a(3) =12a(4) =34a(5) =111a(6) =360a(7) =1226a(8) =4206a(9) =14728a(10) =52024a(11) =185824a(12) =668676a(13) =2424033a(14) =8839632a(15) =32412270a(16) =119410390a(17) =441819444a(18) =1641032536a(19) =6116579352a(20) =22870649308a(21) =85764947502a(22) =322476066224