Number of graded lattices on n nodes.

Terms

a(0) =1a(1) =1a(2) =1a(3) =2a(4) =4a(5) =9a(6) =22a(7) =60a(8) =176a(9) =565a(10) =1980a(11) =7528a(12) =30843a(13) =135248a(14) =630004a(15) =3097780a(16) =15991395a(17) =86267557a(18) =484446620a(19) =2822677523a(20) =17017165987