309,140 research outputs found
Asymptotic Dimension
The asymptotic dimension theory was founded by Gromov in the early 90s. In
this paper we give a survey of its recent history where we emphasize two of its
features: an analogy with the dimension theory of compact metric spaces and
applications to the theory of discrete groups.Comment: Added some remarks about coarse equivalence of finitely generated
groups
Small eigenvalues of surfaces - old and new
We discuss our recent work on small eigenvalues of surfaces. As an
introduction, we present and extend some of the by now classical work of Buser
and Randol and explain novel ideas from articles of S\'evennec, Otal, and
Otal-Rosas which are of importance in our line of thought.Comment: 24 pages, 5 figures, all comments welcom
-covering red and blue points in the plane
We say that a finite set of red and blue points in the plane in general
position can be -covered if the set can be partitioned into subsets of
size , with points of one color and point of the other color, in
such a way that, if at each subset the fourth point is connected by
straight-line segments to the same-colored points, then the resulting set of
all segments has no crossings. We consider the following problem: Given a set
of red points and a set of blue points in the plane in general
position, how many points of can be -covered? and we prove
the following results:
(1) If and , for some non-negative integers and ,
then there are point sets , like -equitable sets (i.e.,
or ) and linearly separable sets, that can be -covered.
(2) If , and the points in are in convex position,
then at least points can be -covered, and this bound is tight.
(3) There are arbitrarily large point sets in general position,
with , such that at most points can be -covered.
(4) If , then at least points of
can be -covered. For , there are too many red points and at
least of them will remain uncovered in any -covering.
Furthermore, in all the cases we provide efficient algorithms to compute the
corresponding coverings.Comment: 29 pages, 10 figures, 1 tabl
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