A variant of the Hales-Jewett Theorem

It was shown by V. Bergelson that any set B with positive upper multiplicative density contains nicely intertwined arithmetic and geometric progressions: For each positive integer k there exist integers a,b,d such that {b(a+id)^j:i,j \in\nhat k}\subset B. In particular one cell of each finite partition of the positive integers contains such configurations. We prove a Hales-Jewett type extension of this partition theorem

Strong characterizing sequences of countable groups

Andr\'as Bir\'o and Vera S\'os prove that for any subgroup $G$ of \T generated freely by finitely many generators there is a sequence $A\subset \N$ such that for all \beta \in \T we have ($\|.\|$ denotes the distance to the nearest integer) $$\beta\in G \Rightarrow \sum_{n\in A} \| n \beta\| < \infty,\quad \quad \quad \beta\notin G \Rightarrow \limsup_{n\in A, n \to \infty} \|n \beta\| > 0.$$ We extend this result to arbitrary countable subgroups of \T. We also show that not only the sum of norms but the sum of arbitrary small powers of these norms can be kept small. Our proof combines ideas from the above article with new methods, involving a filter characterization of subgroups of \T

Martingale Inequalities and Deterministic Counterparts

We study martingale inequalities from an analytic point of view and show that a general martingale inequality can be reduced to a pair of deterministic inequalities in a small number of variables. More precisely, the optimal bound in the martingale inequality is determined by a fixed point of a simple nonlinear operator involving a concave envelope. Our results yield an explanation for certain inequalities that arise in mathematical finance in the context of robust hedging.Comment: 22 page

Sequences and filters of characters characterizing subgroups of compact abelian groups

Let H be a countable subgroup of the metrizable compact abelian group G and f:H -> T=R/Z a (not necessarily continuous) character of H. Then there exists a sequence (chi_n)_n of (continuous) characters of G such that lim_n chi_n(alpha) = f(alpha) for all alpha in H and (chi_n(alpha))_n does not converge whenever alpha in G\H. If one drops the countability and metrizability requirement one can obtain similar results by using filters of characters instead of sequences. Furthermore the introduced methods allow to answer questions of Dikranjan et al.Comment: laTex2e, 10 page