7 research outputs found
Set Systems Containing Many Maximal Chains
The purpose of this short problem paper is to raise an extremal question on
set systems which seems to be natural and appealing. Our question is: which set
systems of a given size maximise the number of -element chains in the
power set ? We will show that for each fixed
there is a family of sets containing
such chains, and that this is asymptotically best possible. For smaller set
systems we are unable to answer the question. We conjecture that a `tower of
cubes' construction is extremal. We finish by mentioning briefly a connection
to an extremal problem on posets and a variant of our question for the grid
graph.Comment: 5 page
Generating all subsets of a finite set with disjoint unions
If X is an n-element set, we call a family G of subsets of X a k-generator
for X if every subset of X can be expressed as a union of at most k disjoint
sets in G. Frein, Leveque and Sebo conjectured that for n > 2k, the smallest
k-generators for X are obtained by taking a partition of X into classes of
sizes as equal as possible, and taking the union of the power-sets of the
classes. We prove this conjecture for all sufficiently large n when k = 2, and
for n a sufficiently large multiple of k when k > 2.Comment: Final version, with some additional explanations added in the proof
Comparable pairs in families of sets
Given a family of subsets of , we say two sets are comparable if or . Sperner's
celebrated theorem gives the size of the largest family without any comparable
pairs. This result was later generalised by Kleitman, who gave the minimum
number of comparable pairs appearing in families of a given size.
In this paper we study a complementary problem posed by Erd\H{o}s and Daykin
and Frankl in the early '80s. They asked for the maximum number of comparable
pairs that can appear in a family of subsets of , a quantity we denote
by . We first resolve an old conjecture of Alon and Frankl, showing
that when . We also obtain more
accurate bounds for for sparse and dense families, characterise the
extremal constructions for certain values of , and sharpen some other known
results.Comment: 18 page