Expand all Collapse all | Results 1 - 25 of 33 |
1. CJM Online first
On a sumset conjecture of ErdÅs ErdÅs conjectured that for any set $A\subseteq \mathbb{N}$
with positive
lower asymptotic density, there are infinite sets $B,C\subseteq
\mathbb{N}$
such that $B+C\subseteq A$. We verify ErdÅs' conjecture in
the case that $A$ has Banach density exceeding $\frac{1}{2}$.
As a consequence, we prove that, for $A\subseteq \mathbb{N}$
with
positive Banach density (a much weaker assumption than positive
lower density), we can find infinite $B,C\subseteq \mathbb{N}$
such
that $B+C$ is contained in the union of $A$ and a translate of
$A$. Both of the aforementioned
results are generalized to arbitrary countable
amenable groups. We also provide a positive solution to ErdÅs'
conjecture for subsets of the natural numbers that are pseudorandom.
Keywords:sumsets of integers, asymptotic density, amenable groups, nonstandard analysis Categories:11B05, 11B13, 11P70, 28D15, 37A45 |
2. CJM Online first
Une propriÃ©tÃ© de domination convexe pour les orbites sturmiennes Let ${\bf x}=(x_0,x_1,\ldots)$ be a $N$-periodic sequence of integers
($N\ge1$), and ${\bf s}$ a sturmian sequence with the same barycenter
(and also $N$-periodic, consequently). It is shown that, for affine
functions $\alpha:\mathbb R^\mathbb N_{(N)}\to\mathbb R$ which are increasing relatively
to some order $\le_2$ on $\mathbb R^\mathbb N_{(N)}$ (the space of all $N$-periodic
sequences), the average of $|\alpha|$ on the orbit of ${\bf x}$ is
greater than its average on the orbit of ${\bf s}$.
Keywords:suite sturmienne, domination convexe, optimisation ergodique Categories:37D35, 49N20, 90C27 |
3. CJM Online first
Hyperspace Dynamics of Generic Maps of the Cantor Space We study the hyperspace dynamics induced from generic continuous maps
and from generic homeomorphisms of the Cantor space, with emphasis on the
notions of Li-Yorke chaos, distributional chaos, topological entropy,
chain continuity, shadowing and recurrence.
Keywords:cantor space, continuous maps, homeomorphisms, hyperspace, dynamics Categories:37B99, 54H20, 54E52 |
4. CJM 2013 (vol 66 pp. 57)
Perfect Orderings on Finite Rank Bratteli Diagrams Given a Bratteli diagram $B$, we study the set $\mathcal O_B$ of all
possible orderings on $B$ and its subset
$\mathcal P_B$ consisting of perfect orderings that produce
Bratteli-Vershik topological dynamical systems (Vershik maps). We
give necessary and sufficient conditions for the ordering $\omega$ to be
perfect. On the other hand, a
wide class of non-simple Bratteli diagrams that do not admit Vershik
maps is explicitly described. In the case of finite rank Bratteli
diagrams, we show that the existence of perfect orderings with a prescribed
number of extreme paths constrains significantly the values of the entries of
the incidence matrices and the structure of the diagram $B$. Our
proofs are based on the new notions of skeletons and
associated graphs, defined and studied in the paper. For a Bratteli
diagram $B$ of rank $k$, we endow the set $\mathcal O_B$ with product
measure $\mu$ and prove that there is some $1 \leq j\leq k$ such that
$\mu$-almost all orderings on $B$ have $j$ maximal and $j$ minimal
paths. If $j$ is strictly greater than the number of minimal
components that $B$ has, then $\mu$-almost all orderings are imperfect.
Keywords:Bratteli diagrams, Vershik maps Categories:37B10, 37A20 |
5. CJM 2013 (vol 65 pp. 1287)
$K$-theory of Furstenberg Transformation Group $C^*$-algebras The paper studies the $K$-theoretic invariants of the crossed product
$C^{*}$-algebras associated with an important family of homeomorphisms
of the tori $\mathbb{T}^{n}$ called Furstenberg transformations.
Using the Pimsner-Voiculescu theorem, we prove that given $n$, the
$K$-groups of those crossed products, whose corresponding $n\times n$
integer matrices are unipotent of maximal degree, always have the same
rank $a_{n}$. We show using the theory developed here that a claim
made in the literature about the torsion subgroups of these $K$-groups
is false. Using the representation theory of the simple Lie algebra
$\frak{sl}(2,\mathbb{C})$, we show that, remarkably, $a_{n}$ has a
combinatorial significance. For example, every $a_{2n+1}$ is just the
number of ways that $0$ can be represented as a sum of integers
between $-n$ and $n$ (with no repetitions). By adapting an argument
of van Lint (in which he answered a question of ErdÅs), a simple,
explicit formula for the asymptotic behavior of the sequence
$\{a_{n}\}$ is given. Finally, we describe the order structure of the
$K_{0}$-groups of an important class of Furstenberg crossed products,
obtaining their complete Elliott invariant using classification
results of H. Lin and N. C. Phillips.
Keywords:$K$-theory, transformation group $C^*$-algebra, Furstenberg transformation, Anzai transformation, minimal homeomorphism, positive cone, minimal homeomorphism Categories:19K14, 19K99, 46L35, 46L80, , 05A15, 05A16, 05A17, 15A36, 17B10, 17B20, 37B05, 54H20 |
6. CJM Online first
Motion in a Symmetric Potential on the Hyperbolic Plane We study the motion of a particle in the hyperbolic plane (embedded in Minkowski space), under the action of a potential that depends only on one variable. This problem is the analogous to the spherical pendulum in a unidirectional force field. However, for the discussion of the hyperbolic plane one has to distinguish three inequivalent cases, depending on the direction of the force field. Symmetry reduction, with respect to groups that are not necessarily compact or even reductive, is carried out by way of Poisson varieties and Hilbert maps. For each case the dynamics is discussed, with special attention to linear potentials.
Keywords:Hamiltonian systems with symmetry, symmetries, non-compact symmetry groups, singular reduction Categories:37J15, 70H33, 70F99, 37C80, 34C14, , 20G20 |
7. CJM 2013 (vol 65 pp. 1384)
Estimates of Hausdorff Dimension for Non-wandering Sets of Higher Dimensional Open Billiards This article concerns a class of open billiards consisting of a finite
number of strictly convex, non-eclipsing obstacles $K$. The
non-wandering set $M_0$ of the billiard ball map is a topological
Cantor set and its Hausdorff dimension has been previously estimated
for billiards in $\mathbb{R}^2$, using well-known techniques. We
extend these estimates to billiards in $\mathbb{R}^n$, and make
various refinements to the estimates. These refinements also allow
improvements to other results. We also show that in many cases, the
non-wandering set is confined to a particular subset of $\mathbb{R}^n$
formed by the convex hull of points determined by period 2
orbits. This allows more accurate bounds on the constants used in
estimating Hausdorff dimension.
Keywords:dynamical systems, billiards, dimension, Hausdorff Categories:37D20, 37D40 |
8. CJM 2012 (vol 65 pp. 553)
Addendum and Erratum to "The Fundamental Group of $S^1$-manifolds" This paper provides an addendum and erratum to L. Godinho and
M. E. Sousa-Dias,
"The Fundamental Group of
$S^1$-manifolds". Canad. J. Math. 62(2010), no. 5, 1082--1098.
Keywords:symplectic reduction; fundamental group Categories:53D19, 37J10, 55Q05 |
9. CJM 2012 (vol 65 pp. 349)
Ergodic Properties of Randomly Coloured Point Sets We provide a framework for studying randomly coloured point sets in a
locally compact, second-countable space on which a
metrisable unimodular group acts continuously and properly.
We first construct and describe an
appropriate dynamical system for uniformly discrete uncoloured point sets. For
point sets of finite local complexity, we
characterise ergodicity geometrically in terms of pattern frequencies.
The general framework allows to incorporate a random
colouring of the point sets. We derive an ergodic theorem for randomly
coloured point sets with finite-range dependencies.
Special attention is paid to the exclusion of exceptional instances for uniquely ergodic
systems. The setup allows for a straightforward application to randomly
coloured graphs.
Keywords:Delone sets, dynamical systems Categories:37B50, 37A30 |
10. CJM 2012 (vol 64 pp. 318)
Cubic Polynomials with Periodic Cycles of a Specified Multiplier We consider cubic polynomials $f(z)=z^3+az+b$ defined over
$\mathbb{C}(\lambda)$, with a marked point of period $N$ and multiplier
$\lambda$. In the case $N=1$, there are infinitely many such objects,
and in the case $N\geq 3$, only finitely many (subject to a mild
assumption). The case $N=2$ has particularly rich structure, and we
are able to describe all such cubic polynomials defined over the field
$\bigcup_{n\geq 1}\mathbb{C}(\lambda^{1/n})$.
Keywords:cubic polynomials, periodic points, holomorphic dynamics Category:37P35 |
11. CJM 2011 (vol 65 pp. 149)
Equicontinuous Delone Dynamical Systems We characterize equicontinuous Delone dynamical systems as those
coming from Delone sets with strongly almost periodic Dirac combs.
Within the class of systems with finite local complexity, the only
equicontinuous systems are then shown to be the crystallographic
ones. On the other hand, within the class without finite local
complexity, we exhibit examples of equicontinuous minimal Delone
dynamical systems that are not crystallographic.
Our results solve the problem posed by Lagarias as to whether a Delone
set whose Dirac comb is strongly almost periodic must be
crystallographic.
Keywords:Delone sets, tilings, diffraction, topological dynamical systems, almost periodic systems Category:37B50 |
12. CJM 2011 (vol 64 pp. 1341)
Bowen Measure From Heteroclinic Points We present a new construction of the entropy-maximizing, invariant
probability measure on a Smale space (the Bowen measure). Our
construction is based on points that are unstably equivalent to one
given point, and stably equivalent to another: heteroclinic points.
The spirit of the construction is similar to Bowen's construction from
periodic points, though the techniques are very different. We also
prove results about the growth rate of certain sets of heteroclinic
points, and about the stable and unstable components of the Bowen
measure. The approach we take is to prove results through direct
computation for the case of a Shift of Finite type, and then use
resolving factor maps to extend the results to more general Smale
spaces.
Keywords:hyperbolic dynamics, Smale space Categories:37D20, 37B10 |
13. CJM 2011 (vol 64 pp. 1058)
Optimal Roughening of Convex Bodies A body moves in a rarefied medium composed of point particles at
rest. The particles make elastic reflections when colliding with the
body surface, and do not interact with each other. We consider a
generalization of Newton's minimal resistance problem: given two
bounded convex bodies $C_1$ and $C_2$ such that $C_1 \subset C_2
\subset \mathbb{R}^3$ and $\partial C_1 \cap \partial C_2 = \emptyset$, minimize the
resistance in the class of connected bodies $B$ such that $C_1 \subset
B \subset C_2$. We prove that the infimum of resistance is zero; that
is, there exist "almost perfectly streamlined" bodies.
Keywords:billiards, shape optimization, problems of minimal resistance, Newtonian aerodynamics, rough surface Categories:37D50, 49Q10 |
14. CJM 2011 (vol 63 pp. 1201)
Resonant Tunneling of Fast Solitons through Large Potential Barriers We rigorously study the resonant tunneling of fast solitons through large
potential barriers for the nonlinear SchrÃ¶dinger equation in
one dimension. Our approach covers the case of general nonlinearities,
both local and Hartree (nonlocal).
Keywords:nonlinear Schroedinger equations, external potential, solitary waves, long time behavior, resonant tunneling Categories:37K40, 35Q55, 35Q51 |
15. CJM 2011 (vol 63 pp. 481)
The Ample Cone for a K3 Surface
In this paper, we give several pictorial fractal
representations of the ample or KÃ¤hler cone for surfaces in a
certain class of $K3$ surfaces. The class includes surfaces
described by smooth $(2,2,2)$ forms in ${\mathbb P^1\times\mathbb P^1\times \mathbb P^1}$ defined over a
sufficiently large number field $K$ that have a line parallel to
one of the axes and have Picard number four. We relate the
Hausdorff dimension of this fractal to the asymptotic growth of
orbits of curves under the action of the surface's group of
automorphisms. We experimentally estimate the Hausdorff dimension
of the fractal to be $1.296 \pm .010$.
Keywords:Fractal, Hausdorff dimension, K3 surface, Kleinian groups, dynamics Categories:14J28, , , , 14J50, 11D41, 11D72, 11H56, 11G10, 37F35, 37D05 |
16. CJM 2010 (vol 62 pp. 1082)
The Fundamental Group of $S^1$-manifolds
We address the problem of computing the fundamental
group of a symplectic $S^1$-manifold for non-Hamiltonian actions on
compact manifolds, and for Hamiltonian actions on non-compact
manifolds with a proper moment map. We generalize known results for
compact manifolds equipped with a Hamiltonian $S^1$-action. Several
examples are presented to illustrate our main results.
Categories:53D20, 37J10, 55Q05 |
17. CJM 2009 (vol 61 pp. 656)
Generalized Polynomials and Mild Mixing An unsettled conjecture of V. Bergelson and I. H\aa land proposes that
if $(X,\alg,\mu,T)$ is an invertible weak mixing measure preserving
system, where $\mu(X)<\infty$, and if $p_1,p_2,\dots ,p_k$ are
generalized polynomials (functions built out of regular polynomials
via iterated use of the greatest integer or floor function) having the
property that no $p_i$, nor any $p_i-p_j$, $i\neq j$, is constant on a
set of positive density, then for any measurable sets
$A_0,A_1,\dots
,A_k$, there exists a zero-density set $E\subset \z$ such that
\[\lim_{\substack{n\to\infty\\ n\not\in E}} \,\mu(A_0\cap T^{p_1(n)}A_1\cap \cdots
\cap T^{p_k(n)}A_k)=\prod_{i=0}^k \mu(A_i).\] We formulate and prove a
faithful version of this conjecture for mildly mixing systems and
partially characterize, in the degree two case, the set of families
$\{ p_1,p_2, \dots ,p_k\}$ satisfying the hypotheses of this theorem.
Categories:37A25, 28D05 |
18. CJM 2008 (vol 60 pp. 975)
An AF Algebra Associated with the Farey Tessellation We associate with the Farey tessellation of the upper
half-plane an
AF algebra $\AA$ encoding the ``cutting sequences'' that define
vertical geodesics.
The Effros--Shen AF algebras arise as quotients
of $\AA$. Using the path algebra model for AF algebras we construct, for
each $\tau \in \big(0,\frac{1}{4}\big]$, projections $(E_n)$ in
$\AA$ such that $E_n E_{n\pm 1}E_n \leq \tau E_n$.
Categories:46L05, 11A55, 11B57, 46L55, 37E05, 82B20 |
19. CJM 2008 (vol 60 pp. 658)
Inverse Pressure Estimates and the Independence of Stable Dimension for Non-Invertible Maps We study the case of an Axiom A holomorphic non-degenerate
(hence non-invertible) map $f\from\mathbb P^2
\mathbb C \to \mathbb P^2 \mathbb C$, where $\mathbb P^2 \mathbb C$
stands for the complex
projective space of dimension 2. Let $\Lambda$ denote a basic set for
$f$ of unstable index 1, and $x$ an arbitrary point of $\Lambda$; we
denote by $\delta^s(x)$ the Hausdorff dimension of $W^s_r(x) \cap
\Lambda$, where $r$ is some fixed positive number and $W^s_r(x)$ is
the local stable manifold at $x$ of size $r$; $\delta^s(x)$ is called
\emph{the stable dimension at} $x$. Mihailescu and
Urba\'nski introduced a notion of inverse topological pressure,
denoted by $P^-$, which takes into consideration preimages of points.
Manning and McCluskey study the case of hyperbolic diffeomorphisms on
real surfaces and give formulas for Hausdorff dimension. Our
non-invertible situation is different here since the local unstable
manifolds are not uniquely determined by their base point, instead
they depend in general on whole prehistories of the base points. Hence
our methods are different and are based on using a sequence of inverse
pressures for the iterates of $f$, in order to give upper and lower
estimates of the stable dimension. We obtain an estimate of the
oscillation of the stable dimension on $\Lambda$. When each point $x$
from $\Lambda$ has the same number $d'$ of preimages in $\Lambda$,
then we show that $\delta^s(x)$ is independent
of $x$; in fact $\delta^s(x)$ is shown to be equal in this case with
the unique zero of the map $t \to P(t\phi^s - \log d')$. We also
prove the Lipschitz continuity of the stable vector spaces over
$\Lambda$; this proof is again different than the one for
diffeomorphisms (however, the unstable distribution is not always
Lipschitz for conformal non-invertible maps). In the end we include
the corresponding results for a real conformal setting.
Keywords:Hausdorff dimension, stable manifolds, basic sets, inverse topological pressure Categories:37D20, 37A35, 37F35 |
20. CJM 2008 (vol 60 pp. 572)
Non-Selfadjoint Perturbations of Selfadjoint Operators in Two Dimensions IIIa. One Branching Point This is the third in a series of works devoted to spectral
asymptotics for non-selfadjoint
perturbations of selfadjoint $h$-pseudodifferential operators in dimension 2, having a
periodic classical flow. Assuming that the strength $\epsilon$
of the perturbation is in the range $h^2\ll \epsilon \ll h^{1/2}$
(and may sometimes reach even smaller values), we
get an asymptotic description of the eigenvalues in rectangles
$[-1/C,1/C]+i\epsilon [F_0-1/C,F_0+1/C]$, $C\gg 1$, when $\epsilon F_0$ is a saddle point
value of the flow average of the leading perturbation.
Keywords:non-selfadjoint, eigenvalue, periodic flow, branching singularity Categories:31C10, 35P20, 35Q40, 37J35, 37J45, 53D22, 58J40 |
21. CJM 2008 (vol 60 pp. 189)
Furstenberg Transformations and Approximate Conjugacy Let $\alpha$ and
$\beta$ be two Furstenberg transformations on $2$-torus associated
with irrational numbers $\theta_1,$ $\theta_2,$ integers $d_1, d_2$ and Lipschitz functions
$f_1$ and $f_2$. It is shown that $\alpha$ and $\beta$ are approximately conjugate in a
measure theoretical sense if (and only
if) $\overline{\theta_1\pm \theta_2}=0$ in $\R/\Z.$ Closely related to the classification of simple
amenable \CAs, it is shown that $\af$ and $\bt$ are approximately $K$-conjugate if (and only if)
$\overline{\theta_1\pm \theta_2}=0$ in $\R/\Z$ and $|d_1|=|d_2|.$ This
is also shown to be equivalent to the condition that the associated crossed product \CAs are isomorphic.
Keywords:Furstenberg transformations, approximate conjugacy Categories:37A55, 46L35 |
22. CJM 2007 (vol 59 pp. 596)
Eigenvalues, $K$-theory and Minimal Flows Let $(Y,T)$ be a minimal suspension flow built over a dynamical
system $(X,S)$ and with (strictly positive, continuous) ceiling
function $f\colon X\to\R$. We show that the eigenvalues of
$(Y,T)$ are contained in the range of a trace on the $K_0$-group
of $(X,S)$. Moreover, a trace gives an order isomorphism of a
subgroup of $K_0(\cprod{C(X)}{S})$ with the group of
eigenvalues of $(Y,T)$. Using this result, we relate the values of
$t$ for which the time-$t$ map on the minimal suspension flow is
minimal with the $K$-theory of the base of this suspension.
Categories:37A55, 37B05 |
23. CJM 2007 (vol 59 pp. 311)
Growth and Zeros of the Zeta Function for Hyperbolic Rational Maps This paper describes new results on the growth and zeros of the Ruelle
zeta function for the Julia set of a hyperbolic rational map. It is
shown that the zeta function is bounded by $\exp(C_K |s|^{\delta})$ in
strips $|\Real s| \leq K$, where $\delta$ is the dimension of the
Julia set. This leads to bounds on the number of zeros in strips
(interpreted as the Pollicott--Ruelle resonances of this dynamical
system). An upper bound on the number of zeros in polynomial regions
$\{|\Real s | \leq |\Imag s|^\alpha\}$ is given, followed by weaker
lower bound estimates in strips $\{\Real s > -C, |\Imag s|\leq r\}$,
and logarithmic neighbourhoods
$\{ |\Real s | \leq \rho \log |\Imag s| \}$.
Recent numerical work of Strain--Zworski suggests the upper
bounds in strips are optimal.
Keywords:zeta function, transfer operator, complex dynamics Category:37C30 |
24. CJM 2006 (vol 58 pp. 39)
$C^*$-Algebras of Irreversible Dynamical Systems We show that certain $C^*$-algebras which have been studied by,
among others, Arzumanian, Vershik, Deaconu, and Renault, in
connection with a measure-preserving transformation of a measure space
or a covering map of a compact space, are special cases of the
endomorphism crossed-product construction recently introduced by the
first named author. As a consequence these algebras are given
presentations in terms of generators and relations. These results
come as a consequence of a general theorem on faithfulness of
representations which are covariant with respect to certain circle
actions. For the case of topologically free covering maps we prove a
stronger result on faithfulness of representations which needs no
covariance. We also give a necessary and sufficient condition for
simplicity.
Categories:46L55, 37A55 |
25. CJM 2005 (vol 57 pp. 1291)
Dupin Hypersurfaces in $\mathbb R^5$ We study Dupin
hypersurfaces in $\mathbb R^5$ parametrized by lines of curvature, with
four distinct principal curvatures. We characterize locally a generic
family of such hypersurfaces in terms of the principal curvatures and
four vector valued functions of one variable. We show that these vector
valued functions are invariant by inversions and homotheties.
Categories:53B25, 53C42, 35N10, 37K10 |