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1. CMB 2012 (vol 56 pp. 709)
Universal Minimal Flows of Groups of Automorphisms of Uncountable Structures It is a well-known fact, that the greatest ambit for
a topological group $G$ is the Samuel compactification of $G$ with
respect to the right uniformity on $G.$ We apply the original
description by Samuel from 1948 to give a simple computation of the
universal minimal flow for groups of automorphisms of uncountable
structures using FraÃ¯ssÃ© theory and Ramsey theory. This work
generalizes some of the known results about countable structures.
Keywords:universal minimal flows, ultrafilter flows, Ramsey theory Categories:37B05, 03E02, 05D10, 22F50, 54H20 |
2. CMB 2011 (vol 55 pp. 410)
A Ramsey Theorem with an Application to Sequences in Banach Spaces The notion of a maximally conditional sequence is introduced for sequences in a Banach space. It is then proved using
Ramsey theory that every basic sequence in a Banach space has a subsequence which is either an unconditional
basic sequence or a maximally conditional sequence. An apparently novel, purely combinatorial lemma in the spirit of
Galvin's theorem is used in the proof. An alternative proof
of the dichotomy result for sequences in Banach spaces is
also sketched,
using the Galvin-Prikry theorem.
Keywords:Banach spaces, Ramsey theory Categories:46B15, 05D10 |
3. CMB 2007 (vol 50 pp. 632)
Transformations and Colorings of Groups Let $G$ be a compact topological group and let $f\colon G\to G$ be a
continuous transformation of $G$. Define $f^*\colon G\to G$ by
$f^*(x)=f(x^{-1})x$ and let $\mu=\mu_G$ be Haar measure on $G$. Assume
that $H=\Imag f^*$ is a subgroup of $G$ and for every
measurable $C\subseteq H$,
$\mu_G((f^*)^{-1}(C))=\mu_H(C)$. Then for every measurable
$C\subseteq G$, there exist $S\subseteq C$ and $g\in G$ such that
$f(Sg^{-1})\subseteq Cg^{-1}$ and $\mu(S)\ge(\mu(C))^2$.
Keywords:compact topological group, continuous transformation, endomorphism, Ramsey theoryinversion, Categories:05D10, 20D60, 22A10 |
4. CMB 1999 (vol 42 pp. 25)
On the Set of Common Differences in van der Waerden's Theorem on Arithmetic Progressions Analogues of van der Waerden's theorem on arithmetic progressions
are considered where the family of all arithmetic progressions,
$\AP$, is replaced by some subfamily of $\AP$. Specifically, we
want to know for which sets $A$, of positive integers, the
following statement holds: for all positive integers $r$ and $k$,
there exists a positive integer $n= w'(k,r)$ such that for every
$r$-coloring of $[1,n]$ there exists a monochromatic $k$-term
arithmetic progression whose common difference belongs to $A$. We
will call any subset of the positive integers that has the above
property {\em large}. A set having this property for a specific
fixed $r$ will be called {\em $r$-large}. We give some necessary
conditions for a set to be large, including the fact that every
large set must contain an infinite number of multiples of each
positive integer. Also, no large set $\{a_{n}: n=1,2,\dots\}$ can
have $\liminf\limits_{n \rightarrow \infty} \frac{a_{n+1}}{a_{n}} > 1$.
Sufficient conditions for a set to be large are also given. We
show that any set containing $n$-cubes for arbitrarily large $n$,
is a large set. Results involving the connection between the
notions of ``large'' and ``2-large'' are given. Several open
questions and a conjecture are presented.
Categories:11B25, 05D10 |
5. CMB 1997 (vol 40 pp. 149)
Monochromatic homothetic copies\\ of $\{1,1+s,1+s+t\}$ For positive integers $s$ and $t$, let $f(s, t)$ denote the smallest positive
integer $N$ such that every $2$-colouring of $[1,N]=\{1,2, \ldots , N\}$ has
a monochromatic homothetic copy of $\{1, 1+s, 1+s+t\}$.
We show that $f(s, t) = 4(s+t) + 1$ whenever $s/g$ and $t/g$ are not
congruent to $0$ (modulo $4$), where $g=\gcd(s,t)$. This can be viewed as
a generalization of part of van~der~Waerden's theorem on
arithmetic progressions, since the $3$-term arithmetic progressions are the
homothetic copies of $\{1, 1+1, 1+1+1\}$. We also show that $f(s, t) = 4(s+t)
+ 1$ in many other cases (for example, whenever $s > 2t > 2$ and $t$ does not
divide $s$), and that $f(s, t) \le 4(s+t) + 1$ for all $s$, $t$.
Thus the set of homothetic copies of $\{1, 1+s, 1+s+t\}$ is a set of
triples with a particularly simple Ramsey function (at least for the case
of two colours), and one wonders what other ``natural'' sets of triples,
quadruples, {\it etc.}, have simple (or easily estimated) Ramsey functions.
Category:05D10 |