16 research outputs found
Mapping grid points onto a square forces an arbitrarily large Lipschitz constant
We prove that the regular square grid of points in the integer
lattice cannot be recovered from an arbitrary -element
subset of via a mapping with prescribed Lipschitz constant
(independent of ). This answers negatively a question of Feige from 2002.
Our resolution of Feige's question takes place largely in a continuous setting
and is based on some new results for Lipschitz mappings falling into two broad
areas of interest, which we study independently. Firstly the present work
contains a detailed investigation of Lipschitz regular mappings on Euclidean
spaces, with emphasis on their bilipschitz decomposability in a sense
comparable to that of the well known result of Jones. Secondly, we build on
work of Burago and Kleiner and McMullen on non-realisable densities. We verify
the existence, and further prevalence, of strongly non-realisable densities
inside spaces of continuous functions.Comment: 60 pages (43 pages of the main part, 13 pages of appendices), 10
figures. This is a revised version according to referees' comments. Our
version of the proof of the theorem about bilipschitz decomposition of
Lipschitz regular mappings was greatly simplified. To appear in GAF
Even maps, the Colin de~Verdi\`ere number and representations of graphs
Van der Holst and Pendavingh introduced a graph parameter , which
coincides with the more famous Colin de Verdi\`{e}re graph parameter for
small values. However, the definition of is much more
geometric/topological directly reflecting embeddability properties of the
graph. They proved and conjectured for any graph . We confirm this conjecture. As far as we know,
this is the first topological upper bound on which is, in general,
tight.
Equality between and does not hold in general as van der Holst
and Pendavingh showed that there is a graph with and
. We show that the gap appears on much smaller values,
namely, we exhibit a graph for which and .
We also prove that, in general, the gap can be large: The incidence graphs
of finite projective planes of order satisfy and .Comment: 28 pages, 4 figures. In v2 we slightly changed one of the core
definitions (previously "extended representation" now "semivalid
representation"). We also use it to introduce a new graph parameter, denoted
eta, which did not appear in v1. It allows us to establish an extended
version of the main result showing that mu(G) is at most eta(G) which is at
most sigma(G) for every graph
Shortest path embeddings of graphs on surfaces
The classical theorem of F\'{a}ry states that every planar graph can be
represented by an embedding in which every edge is represented by a straight
line segment. We consider generalizations of F\'{a}ry's theorem to surfaces
equipped with Riemannian metrics. In this setting, we require that every edge
is drawn as a shortest path between its two endpoints and we call an embedding
with this property a shortest path embedding. The main question addressed in
this paper is whether given a closed surface S, there exists a Riemannian
metric for which every topologically embeddable graph admits a shortest path
embedding. This question is also motivated by various problems regarding
crossing numbers on surfaces.
We observe that the round metrics on the sphere and the projective plane have
this property. We provide flat metrics on the torus and the Klein bottle which
also have this property.
Then we show that for the unit square flat metric on the Klein bottle there
exists a graph without shortest path embeddings. We show, moreover, that for
large g, there exist graphs G embeddable into the orientable surface of genus
g, such that with large probability a random hyperbolic metric does not admit a
shortest path embedding of G, where the probability measure is proportional to
the Weil-Petersson volume on moduli space.
Finally, we construct a hyperbolic metric on every orientable surface S of
genus g, such that every graph embeddable into S can be embedded so that every
edge is a concatenation of at most O(g) shortest paths.Comment: 22 pages, 11 figures: Version 3 is updated after comments of
reviewer
Metric and analytic methods
The thesis deals with two separate problems. In the first part we show that the regular n×n grid of points in Z2 cannot be recovered from an arbitrary n2 -element subset of Z2 using only mappings with prescribed maximum stretch independent of n. This provides a negative answer to a question of Uriel Feige from 2002. The present approach builds on the work of Burago and Kleiner and McMullen from 1998 on bilipschitz non-realisable densities and bilipschitz non-equivalence of separated nets in the plane. We describe a procedure that takes a positive, measurable function and encodes it into a sequence of discrete sets. Then we show that applying this procedure to a typical positive, continuous function on the unit square yields a counter-example to Feige's question. Along the way we provide a new proof of a result on bilipschitz decomposition for Lipschitz regular mappings, which was originally proved by Bonk and Kleiner in 2002. In the second part we provide a constructive proof for the strong Hanani- Tutte theorem on the projective plane. In contrast to the previous proof by Pelsmajer, Schaefer and Stasi from 2009, the presented approach does not rely on characterisation of embeddability into the projective plane via forbidden minors.
Lipschitz mappings of discrete sets
In this thesis we consider Feige's question of whether there always exists a constantly Lipschitz bijection of an n2 -element set S ⊂ Z2 onto a regular lattice of n × n points in Z2 . We propose a solution of this problem in case the points of the set S form a long rectangle or they are arranged in the shape of a square without a part of its interior points. The main part is a summary of Burago's and Kleiner's article [2] and the article by McMullen [12] dealing with the problem of existence of separated nets in R2 that are not bi-Lipschitz equivalent to the integer lattice. This problem looks similar to Feige's problem. According to these articles we construct a separated net that is not bi-Lipschitz equivalent to the integer lattice, using a positive bounded measurable function that is not the Jacobian of a bi-Lipschitz homeomorphism almost everywhere. We present McMullen's construction of such a function and we complete his proof of its correctness.
Lipschitz mappings in the plane
In this thesis we consider an open question of Feige that asks whether there always exists a constantly Lipschitz bijection of an n2 -point subset of Z2 onto a regular grid [n] × [n] for every n ∈ N. We relate this question to an already resolved problem of the existence of a bounded positive measurable density in R2 that is not the Jacobian of any bilipschitz map. This problem was resolved by Burago and Kleiner [1], and independently, by McMullen [12]. We present the work of Burago and Kleiner, analyze its relation to Feige's problem and sug- gest a continuous formulation of Feige's question in a special case. Then we present the Burago-Kleiner density, make several observation about the properties of this density, and after that we construct a density that is everywhere nonrealizable as the Jacobian of a bilipschitz map. Subsequently, we discuss our continuous variant of Feige's question, provide several observation concerning it, and finally, we try to use the everywhere nonrealizable density constructed before to answer our continuous variant of Feige's question. However, this last task still remains incomplete.
Metric and analytic methods
The thesis deals with two separate problems. In the first part we show that the regular n×n grid of points in Z2 cannot be recovered from an arbitrary n2 -element subset of Z2 using only mappings with prescribed maximum stretch independent of n. This provides a negative answer to a question of Uriel Feige from 2002. The present approach builds on the work of Burago and Kleiner and McMullen from 1998 on bilipschitz non-realisable densities and bilipschitz non-equivalence of separated nets in the plane. We describe a procedure that takes a positive, measurable function and encodes it into a sequence of discrete sets. Then we show that applying this procedure to a typical positive, continuous function on the unit square yields a counter-example to Feige's question. Along the way we provide a new proof of a result on bilipschitz decomposition for Lipschitz regular mappings, which was originally proved by Bonk and Kleiner in 2002. In the second part we provide a constructive proof for the strong Hanani- Tutte theorem on the projective plane. In contrast to the previous proof by Pelsmajer, Schaefer and Stasi from 2009, the presented approach does not rely on characterisation of embeddability into the projective plane via forbidden minors.
Lipschitz mappings of discrete sets
In this thesis we consider Feige's question of whether there always exists a constantly Lipschitz bijection of an n2 -element set S ⊂ Z2 onto a regular lattice of n × n points in Z2 . We propose a solution of this problem in case the points of the set S form a long rectangle or they are arranged in the shape of a square without a part of its interior points. The main part is a summary of Burago's and Kleiner's article [2] and the article by McMullen [12] dealing with the problem of existence of separated nets in R2 that are not bi-Lipschitz equivalent to the integer lattice. This problem looks similar to Feige's problem. According to these articles we construct a separated net that is not bi-Lipschitz equivalent to the integer lattice, using a positive bounded measurable function that is not the Jacobian of a bi-Lipschitz homeomorphism almost everywhere. We present McMullen's construction of such a function and we complete his proof of its correctness.