arXiv subjects

Alessandro Codenotti

Publications and source records attributed to Alessandro Codenotti.

6 recordsLinked to original sources

arXiv2025

Fixed points on null and tame flows for groups of automorphisms

Using a generalization of the Kechris-Pestov-Todor\v{c}evi\'{c} correspondence due to Nguyen Van Th\'{e} we obtain fixed point theorems for null and tame actions of groups of the form $\mathrm{Aut}(\mathcal F)$, where $\mathcal{F}$ is a Fra\"{i}ss\'{e} structure. In particular we show that if $\mathrm{Age}(\mathcal F)$ is a free joint embedding class, then every null flow $\mathrm{Aut}(\mathcal F)\curvearrowright X$ has a fixed point, while if $\mathrm{Age}(\mathcal F)$ is a free amalgamation class, then every tame flow $\mathrm{Aut}(\mathcal F)\curvearrowright X$ has a fixed point.

math.LO↗

arXiv2024

Homological algebra of pro-Lie Polish abelian groups

In this paper, we initiate the study of pro-Lie Polish abelian groups from the perspective of homological algebra. We extend to this context the type-decomposition of locally compact Polish abelian groups of Hoffmann and Spitzweck, and prove that the category $\mathbf{proLiePAb}$ of pro-Lie Polish abelian groups is a thick subcategory of the category of Polish abelian groups. We completely characterize injective and projective objects in $\mathbf{proLiePAb}$. We conclude that $\mathbf{proLiePAb}$ has enough projectives but not enough injectives and homological dimension $1$. We also completely characterize injective and projective objects in the category of non-Archimedean Polish abelian groups, concluding that it has enough injectives and projectives and homological dimension $1$. Injective objects are also characterized for the categories of topological torsion Polish abelian groups and for Polish abelian topological $p$-groups, showing that these categories have enough injectives and homological dimension $1$.

math.AC↗

arXiv2024

Surfaces and other Peano Continua with no Generic Chains

The space of chains on a compact connected space encodes all the different ways of continuously growing out of a point until exhausting the space. A chain is \emph{generic} if its orbit under the action of the underlying homeomorphism group is comeager. In this paper we show that a large family of topological spaces do not have a generic chain: in addition to all manifolds of dimension at least 3, for which the result was already known, our theorem covers all compact surfaces except for the sphere and the real projective plane - for which the question remains open - as well as all other homogeneous Peano continua, circle excluded. If the spaces are moreover strongly locally homogeneous, which is the case for any closed manifold and the Menger curve, we prove that chains cannot be classified up to homeomorphism by countable structures, and that the underlying homeomorphism groups have non-metrizable universal minimal flows, with all orbits meager, in contrast to the case of 1-dimensional manifolds. The proof of the main result is of combinatorial nature, and it relies on the creation of a dictionary between open sets of chains on one side, and walks on finite connected graphs on the other.

math.DS↗

arXiv2023

Ranks in Ellis semigroups and model theory

We slightly generalize a notion of rank introduced by Glasner and Megrelishvili, which captures the oscillations of elements of Ellis semigroups, so that it can be applied to any compact Hausdorff space instead of being limited to the metric case. Then, we relate this rank to classical dividing lines in the model-theoretic stability hierarchy. For example, that the rank is ordinal-valued if and only if the background theory is NIP.

math.LO↗

arXiv2023

Some examples of tame dynamical systems answering questions of Glasner and Megrelishvili

Glasner and Megrelishvili proved that every continuous action of a topological group $G$ on a dendrite $X$ is tame. We produce two examples of an action on a dendrite which is not $\mathrm{tame}_1$, answering a question they raised. We then show that actions on dendrites have $\beta$-rank at most $2$ and produce examples of tame metric dynamical systems of $\beta$-rank $\alpha$ for any $\alpha<\omega_1$, answering another question of Glasner and Megrelishvili.

math.DS↗

arXiv2022

Projective Fra\"{i}ss\'{e} limits and generalized Wa\.{z}ewski dendrites

We continue the study of projective Fra\"{i}ss\'{e} limits of trees initiated by Charatonik and Roe and we construct many generalized Wa\.{z}ewski dendrites as the topological realization of a projective Fra\"{i}ss\'{e} limit of families of finite trees with (weakly) coherent epimorphisms. Moreover we use the categorical approach to Fra\"{i}ss\'{e} limits developed by Kubi\'{s} to construct all generalized Wa\.{z}ewski dendrites as topological realizations of Fra\"{i}ss\'{e} limits of suitable categories of finite structures. As an application we recover a homogeneity result for countable dense sets of endpoints in generalized Wa\.{z}ewski dendrites.

math.LO↗