arXiv subjects

Tristán Radić

Publications and source records attributed to Tristán Radić.

7 recordsLinked to original sources

arXiv2026

Infinite prime sumsets in structured and $U^k(Φ)$-uniform sets

By introducing new ergodic-theoretic techniques in nilsystems, we determine which infinite sumset configurations occur in $U^k(Φ)$-uniform and Nil-Bohr sets. To be more precise, our first result associates the degree $k$ of a $U^k(Φ)$-uniform set with the variety of sumsets it contains, solving a conjecture of Kra, Moreira, Richter and Robertson. Restricting to Nil-Bohr sets we show the existence of infinite sumsets with summands in the shifted primes $\mathbb{P}-1$. As a consequence, we show that for any real polynomial $Q(n)$ with leading irrational coefficient of degree $k$, and any natural numbers $\ell_1, \cdots, \ell_k$ there is an infinite set $P\subset \mathbb{P}$ such that \begin{equation*} Q\Big(\sum_{p \in I} p\Big) \in U \pmod 1 \quad \text{ for all } I \subset P , |I| = \ell_1, \ldots, \ell_k. \end{equation*}

math.DS↗

arXiv2026

Infinite sumsets in $U^k(Φ)$-uniform sets

Extending recent developments of Kra, Moreira, Richter and Roberson, we study infinite sumset patterns in $U^k(Φ)$-uniform subsets of the integers, defined via the local uniformity seminorms introduced by Host and Kra. We relate the degree $k$ of a $U^k(Φ)$-uniform set to the existence of a rich variety of sumset patterns. As a counterpart, we stablish higher order parity obstruction to sumsets arising from nilsystems. We also provide examples of $U^k(Φ)$-uniform sets for applications, including sets arising from the Thue-Morse and Rudin-Shapiro sequences.

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arXiv2025

On density analogs of Hindman's finite sums theorem

For any set $A$ of natural numbers with positive upper Banach density, we show the existence of an infinite set $B$ and sequences $(t_k)_{k\in \mathbb{N}}, (s_k)_{k\in \mathbb{N}}$ of natural numbers such that $\left\{ \sum_{n \in F}n : F \subset B + s_k, 1 \leq |F| \leq k \right\}\subset A-t_k$, for every $k\in \mathbb{N}$. This strengthens the density finite sums theorem of Kra, Moreira, Richter, and Robertson. We further show, given such a set $A$, the existence of an infinite set $B$ and a sequence $(t_k)_{k\in \mathbb{N}}$ of natural numbers such that $\left\{ \sum_{n \in F}n : F \subset B, |F| = k \right\}\subset A-t_k$, for every $k\in \mathbb{N}$. As a corollary, we obtain a sequence $(B_n)_{n\in \mathbb{N}}$ of infinite sets of natural numbers such that $B_1+\cdots +B_k \subset A$, for every $k\in \mathbb{N}$. We also establish the optimality of our main theorems by providing counterexamples to potential further generalizations, and thereby addressing questions of the aforementioned authors in the context of density analogs to Hindman's finite sums theorem.

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arXiv2025

Infinite unrestricted sumsets in subsets of abelian groups with large density

Let $(G,+)$ be a countable abelian group such that the subgroup $\{g+g\colon g\in G\}$ has finite index and the doubling map $g\mapsto g+g$ has finite kernel. We establish lower bounds on the upper density of a set $A\subset G$ with respect to an appropriate Følner sequence, so that $A$ contains a sumset of the form $\{t+b_1+b_2\colon b_1,b_2\in B\}$ or $\{b_1+b_2\colon b_1,b_2\in B\}$, for some infinite $B\subset G$ and some $t\in G$. Both assumptions on $G$ are necessary for our results to be true. We also characterize the Følner sequences for which this is possible. Finally, we show that our lower bounds are optimal in a strong sense.

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arXiv2025

On partial rigidity of $\mathcal{S}$-adic subshifts

We develop combinatorial tools to study partial rigidity within the class of minimal $\mathcal{S}$-adic subshifts. By leveraging the combinatorial data of well-chosen Kakutani-Rokhlin partitions, we establish a necessary and sufficient condition for partial rigidity. Additionally, we provide an explicit expression to compute the partial rigidity rate and an associated partial rigidity sequence. As applications, we compute the partial rigidity rate for a variety of constant length substitution subshifts, such as the Thue-Morse subshift, where we determine a partial rigidity rate of 2/3. We also exhibit non-rigid substitution subshifts with partial rigidity rates arbitrarily close to 1 and as a consequence, using products of the aforementioned substitutions, we obtain that any number in $[0, 1]$ is the partial rigidity rate of a system.

math.DS↗

arXiv2024

Multiple partial rigidity rates in low complexity subshifts

Partial rigidity is a quantitative notion of recurrence and provides a global obstruction which prevents the system from being strongly mixing. A dynamical system $(X, \mathcal{X}, μ, T)$ is partially rigid if there is a constant $δ>0$ and sequence $(n_k)_{k \in \mathbb{N}}$ such that $\displaystyle \liminf_{k \to \infty } μ(A \cap T^{n_k}A) \geq δμ(A)$ for every $A \in \mathcal{X}$, and the partial rigidity rate is the largest $δ$ achieved over all sequences. For every integer $d \geq 1$, via an explicit construction, we prove the existence of a minimal subshift $(X,S)$ with $d$ ergodic measures having distinct partial rigidity rates. The systems built are $\mathcal{S}$-adic subshifts of finite alphabetic rank that have non-superlinear word complexity and, in particular, have zero entropy.

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arXiv2024

Infinite unrestricted sumsets of the form $B+B$ in sets with large density

For a set $A \subset \mathbb{N}$ we characterize in terms of its density when there exists an infinite set $B \subset \mathbb{N}$ and $t \in \{0,1\}$ such that $B+B \subset A-t$, where $B+B : =\{b_1+b_2\colon b_1,b_2 \in B\}$. Specifically, when the lower density $\underline{d}(A) >1/2$ or the upper density $\overline{d}(A)> 3/4$, the existence of such a set $B\subset \mathbb{N}$ and $t\in \{0,1\}$ is assured. Furthermore, whenever $\underline{d}(A) > 3/4$ or $\overline{d}(A)>5/6$, we show that the shift $t$ is unnecessary and we also provide examples to show that these bounds are sharp. Finally, we construct a syndetic three-coloring of the natural numbers that does not contain a monochromatic $B+B+t$ for any infinite set $B \subset \mathbb{N}$ and number $t \in \mathbb{N}$.

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