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Search: MSC category 28D15 ( General groups of measure-preserving transformations )

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1. CJM Online first

Di Nasso, Mauro; Goldbring, Isaac; Jin, Renling; Leth, Steven; Lupini, Martino; Mahlburg, Karl
On a sumset conjecture of Erdős
Erdős 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 Erdős' 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 Erdős' 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 2000 (vol 52 pp. 332)

del Junco, Andrés; Yassawi, Reem
Multiple Mixing and Rank One Group Actions
Suppose $G$ is a countable, Abelian group with an element of infinite order and let ${\cal X}$ be a mixing rank one action of $G$ on a probability space. Suppose further that the F\o lner sequence $\{F_n\}$ indexing the towers of ${\cal X}$ satisfies a ``bounded intersection property'': there is a constant $p$ such that each $\{F_n\}$ can intersect no more than $p$ disjoint translates of $\{F_n\}$. Then ${\cal X}$ is mixing of all orders. When $G={\bf Z}$, this extends the results of Kalikow and Ryzhikov to a large class of ``funny'' rank one transformations. We follow Ryzhikov's joining technique in our proof: the main theorem follows from showing that any pairwise independent joining of $k$ copies of ${\cal X}$ is necessarily product measure. This method generalizes Ryzhikov's technique.

Category:28D15

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