Vertically indecomposable lattices on n unlabeled nodes.

Terms

a(0) =1a(1) =1a(2) =1a(3) =0a(4) =1a(5) =2a(6) =7a(7) =27a(8) =126a(9) =664a(10) =3954a(11) =26190a(12) =190754a(13) =1514332a(14) =12998035a(15) =119803771a(16) =1178740932a(17) =12316480222a(18) =136060611189