OFFSET
0,4
EXAMPLE
The a(1) = 1 through a(8) = 19 antichains:
{{1}} {{2}} {{3}} {{4}} {{5}} {{6}}
{{1,2}} {{1,3}} {{1,4}} {{1,5}}
{{1},{2}} {{1},{3}} {{2,3}} {{2,4}}
{{1},{4}} {{1,2,3}}
{{2},{3}} {{1},{5}}
{{2},{4}}
{{1},{2,3}}
{{2},{1,3}}
{{3},{1,2}}
{{1},{2},{3}}
.
{{7}} {{8}}
{{1,6}} {{1,7}}
{{2,5}} {{2,6}}
{{3,4}} {{3,5}}
{{1,2,4}} {{1,2,5}}
{{1},{6}} {{1,3,4}}
{{2},{5}} {{1},{7}}
{{3},{4}} {{2},{6}}
{{1},{2,4}} {{3},{5}}
{{2},{1,4}} {{1},{2,5}}
{{4},{1,2}} {{1},{3,4}}
{{1,2},{1,3}} {{2},{1,5}}
{{1},{2},{4}} {{3},{1,4}}
{{4},{1,3}}
{{5},{1,2}}
{{1,2},{1,4}}
{{1,2},{2,3}}
{{1},{2},{5}}
{{1},{3},{4}}
MATHEMATICA
sps[{}]:={{}}; sps[set:{i_, ___}]:=Join@@Function[s, Prepend[#, s]&/@sps[Complement[set, s]]]/@Cases[Subsets[set], {i, ___}];
mps[set_]:=Union[Sort[Sort/@(#/.x_Integer:>set[[x]])]&/@sps[Range[Length[set]]]];
submultisetQ[M_, N_]:=Or[Length[M]==0, MatchQ[{Sort[List@@M], Sort[List@@N]}, {{x_, Z___}, {___, x_, W___}}/; submultisetQ[{Z}, {W}]]];
antiQ[s_]:=Select[Tuples[s, 2], And[UnsameQ@@#, submultisetQ@@#]&]=={};
Table[Length[Select[Join@@mps/@IntegerPartitions[n], And[UnsameQ@@#, And@@UnsameQ@@@#, antiQ[#]]&]], {n, 10}]
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Gus Wiseman, Oct 12 2018
STATUS
approved