Expansion of c(x)*c(x*c(x)) where c(x) is the g.f. of A000108.

Terms

a(0) =1a(1) =2a(2) =6a(3) =21a(4) =80a(5) =322a(6) =1348a(7) =5814a(8) =25674a(9) =115566a(10) =528528a(11) =2449746a(12) =11485068a(13) =54377288a(14) =259663576a(15) =1249249981a(16) =6049846848a(17) =29469261934a(18) =144293491564a(19) =709806846980