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A326566 Number of covering antichains of subsets of {1..n} with equal edge-sums. 7
2, 1, 1, 2, 4, 14, 92, 1320, 73584, 51913039 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,1

COMMENTS

An antichain is a finite set of finite sets, none of which is a subset of any other. It is covering if its union is {1..n}. The edge-sums are the sums of vertices in each edge, so for example the edge sums of {{1,3},{2,5},{3,4,5}} are {4,7,12}.

LINKS

Table of n, a(n) for n=0..9.

EXAMPLE

The a(1) = 1 through a(5) = 14 antichains:

  {{1}}  {{1,2}}  {{1,2,3}}    {{1,2,3,4}}      {{1,2,3,4,5}}

                  {{3},{1,2}}  {{1,4},{2,3}}    {{1,2,5},{1,3,4}}

                               {{2,4},{1,2,3}}  {{1,3,5},{2,3,4}}

                               {{3,4},{1,2,4}}  {{1,4,5},{2,3,5}}

                                                {{5},{1,4},{2,3}}

                                                {{1,4,5},{1,2,3,4}}

                                                {{2,3,5},{1,2,3,4}}

                                                {{2,4,5},{1,2,3,5}}

                                                {{3,4,5},{1,2,4,5}}

                                                {{1,5},{2,4},{1,2,3}}

                                                {{2,5},{3,4},{1,2,4}}

                                                {{3,5},{1,2,5},{1,3,4}}

                                                {{4,5},{1,3,5},{2,3,4}}

                                                {{1,4,5},{2,3,5},{1,2,3,4}}

MATHEMATICA

stableSets[u_, Q_]:=If[Length[u]==0, {{}}, With[{w=First[u]}, Join[stableSets[DeleteCases[u, w], Q], Prepend[#, w]&/@stableSets[DeleteCases[u, r_/; r==w||Q[r, w]||Q[w, r]], Q]]]];

cleq[n_]:=Select[stableSets[Subsets[Range[n]], SubsetQ[#1, #2]||Total[#1]!=Total[#2]&], Union@@#==Range[n]&];

Table[Length[cleq[n]], {n, 0, 5}]

CROSSREFS

The non-covering case is A326574.

Cf. A000372, A006126, A035470, A307249, A321455, A321717, A321718, A326518, A326534, A326565.

Sequence in context: A024957 A153914 A151893 * A213953 A000361 A246596

Adjacent sequences:  A326563 A326564 A326565 * A326567 A326568 A326569

KEYWORD

nonn,more

AUTHOR

Gus Wiseman, Jul 13 2019

EXTENSIONS

a(9) from Andrew Howroyd, Aug 14 2019

STATUS

approved

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Last modified November 27 05:27 EST 2020. Contains 338678 sequences. (Running on oeis4.)