169 research outputs found
Cross-intersecting families of vectors
Given a sequence of positive integers , let
denote the family of all sequences of positive integers
such that for all . Two families of sequences (or vectors),
, are said to be -cross-intersecting if no matter how we
select and , there are at least distinct indices
such that . We determine the maximum value of over all
pairs of - cross-intersecting families and characterize the extremal pairs
for , provided that . The case is
quite different. For this case, we have a conjecture, which we can verify under
additional assumptions. Our results generalize and strengthen several previous
results by Berge, Frankl, F\"uredi, Livingston, Moon, and Tokushige, and
answers a question of Zhang
A constructive proof of the general Lovasz Local Lemma
The Lovasz Local Lemma [EL75] is a powerful tool to non-constructively prove
the existence of combinatorial objects meeting a prescribed collection of
criteria. In his breakthrough paper [Bec91], Beck demonstrated that a
constructive variant can be given under certain more restrictive conditions.
Simplifications of his procedure and relaxations of its restrictions were
subsequently exhibited in several publications [Alo91, MR98, CS00, Mos06,
Sri08, Mos08]. In [Mos09], a constructive proof was presented that works under
negligible restrictions, formulated in terms of the Bounded Occurrence
Satisfiability problem. In the present paper, we reformulate and improve upon
these findings so as to directly apply to almost all known applications of the
general Local Lemma.Comment: 8 page
Conflict-free coloring of graphs
We study the conflict-free chromatic number chi_{CF} of graphs from extremal
and probabilistic point of view. We resolve a question of Pach and Tardos about
the maximum conflict-free chromatic number an n-vertex graph can have. Our
construction is randomized. In relation to this we study the evolution of the
conflict-free chromatic number of the Erd\H{o}s-R\'enyi random graph G(n,p) and
give the asymptotics for p=omega(1/n). We also show that for p \geq 1/2 the
conflict-free chromatic number differs from the domination number by at most 3.Comment: 12 page
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