OFFSET
0,4
COMMENTS
A set system (set of sets) is an antichain if no element is a subset of any other.
LINKS
Dmitry I. Ignatov, On the Number of Maximal Antichains in Boolean Lattices for n up to 7. Lobachevskii J. Math., 44 (2023), 137-146.
Dmitry I. Ignatov, Supporting iPython code and input files for a(7) based on inequivalent maximal antichains for n=7 and related sequences, Github repository, section 3
Dmitry I. Ignatov, PDF version of the supporting iPython notebook for a(7)
Dmitry I. Ignatov, Supporting iPython notebook for a(7): A326360.ipynb
FORMULA
a(n) = Sum_{k=0..n} (-1)^(n-k)*binomial(n,k)*A326359(k) for n >= 2. - Andrew Howroyd, Nov 19 2021
EXAMPLE
The a(1) = 1 through a(4) = 13 maximal antichains:
{} {12} {123} {1234}
{12}{13}{23} {12}{134}{234}
{13}{124}{234}
{14}{123}{234}
{23}{124}{134}
{24}{123}{134}
{34}{123}{124}
{12}{13}{14}{234}
{12}{23}{24}{134}
{13}{23}{34}{124}
{14}{24}{34}{123}
{123}{124}{134}{234}
{12}{13}{14}{23}{24}{34}
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]]]];
fasmax[y_]:=Complement[y, Union@@(Most[Subsets[#]]&/@y)];
Table[Length[fasmax[stableSets[Subsets[Range[n], {2, n}], SubsetQ]]], {n, 0, 4}]
PROG
CROSSREFS
Antichains of nonempty, non-singleton sets are A307249.
Minimal covering antichains are A046165.
Maximal intersecting antichains are A007363.
Maximal antichains of nonempty sets are A326359.
KEYWORD
nonn,more
AUTHOR
Gus Wiseman, Jul 01 2019
EXTENSIONS
a(6) from Andrew Howroyd, Aug 14 2019
a(7) from Dmitry I. Ignatov, Oct 14 2021
STATUS
approved