694 research outputs found
On the Duality of Semiantichains and Unichain Coverings
We study a min-max relation conjectured by Saks and West: For any two posets
and the size of a maximum semiantichain and the size of a minimum
unichain covering in the product are equal. For positive we state
conditions on and that imply the min-max relation. Based on these
conditions we identify some new families of posets where the conjecture holds
and get easy proofs for several instances where the conjecture had been
verified before. However, we also have examples showing that in general the
min-max relation is false, i.e., we disprove the Saks-West conjecture.Comment: 10 pages, 3 figure
An extremal problem on crossing vectors
For positive integers and , two vectors and from
are called -crossing if there are two coordinates and
such that and . What is the maximum size of
a family of pairwise -crossing and pairwise non--crossing vectors in
? We state a conjecture that the answer is . We prove
the conjecture for and provide weaker upper bounds for .
Also, for all and , we construct several quite different examples of
families of desired size . This research is motivated by a natural
question concerning the width of the lattice of maximum antichains of a
partially ordered set.Comment: Corrections and improvement
Antichain cutsets of strongly connected posets
Rival and Zaguia showed that the antichain cutsets of a finite Boolean
lattice are exactly the level sets. We show that a similar characterization of
antichain cutsets holds for any strongly connected poset of locally finite
height. As a corollary, we get such a characterization for semimodular
lattices, supersolvable lattices, Bruhat orders, locally shellable lattices,
and many more. We also consider a generalization to strongly connected
hypergraphs having finite edges.Comment: 12 pages; v2 contains minor fixes for publicatio
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