Failure of Khintchine-type results along the polynomial image of IP$_0$ sets
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| Format: | Preprint |
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2023
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| _version_ | 1866910290418860032 |
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| author | Zelada, Rigoberto |
| author_facet | Zelada, Rigoberto |
| contents | In "IP-sets and polynomial recurrence", Bergelson, Furstenberg, and McCutcheon established the following far reaching extension of Khintchine's recurrence theorem:
For any invertible probability preserving system $(X,\mathcal A,μ,T)$, any non-constant polynomial $p\in\mathbb Z[x]$ with $p(0)=0$, any $A\in\mathcal A$, and any $ε>0$, the set $$R_ε^p(A)=\{n\in\mathbb N\,|\,μ(A\cap T^{-p(n)}A)>μ^2(A)-ε\}$$ is IP$^*$, meaning that for any increasing sequence $(n_k)_{k\in\mathbb N}$ in $\mathbb N$, $$\text{FS}((n_k)_{k\in\mathbb N})\cap R_ε^p(A)\neq \emptyset,$$ where $$\text{FS}((n_k)_{k\in\mathbb N})=\{\sum_{j\in F}n_j\,|\,F\subseteq \mathbb N\,\text{ is finite}\text{ and }F\neq\emptyset\}=\{n_{k_1}+\cdots+n_{k_t}\,|\,k_1<\cdots<k_t,\,t\in\mathbb N\}.$$ In view of the potential new applications to combinatorics, this result has led to the question of whether a further strengthening of Khintchine's recurrence theorem holds, namely whether the set $R_ε^p(A)$ is IP$_0^*$ meaning that there exists a $t\in\mathbb N$ such that for any finite sequence $n_1<\cdots<n_t$ in $\mathbb N$, $$\{\sum_{j\in F}n_j\,|\,F\subseteq \{1,...,t\}\text{ and }F\neq \emptyset\}\cap R_ε^p(A)\neq \emptyset.$$ In this paper we give a negative answer to this question by showing that for any given polynomial $p\in\mathbb Z[x]$ with deg$(p)>1$ and $p(0)=0$ there is an invertible probability preserving system $(X,\mathcal A,μ,T)$, a set $A\in\mathcal A$, and an $ε>0$ for which the set $R_ε^p(A)$ is not IP$_0^*$. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2307_08764 |
| institution | arXiv |
| publishDate | 2023 |
| record_format | arxiv |
| spellingShingle | Failure of Khintchine-type results along the polynomial image of IP$_0$ sets Zelada, Rigoberto Dynamical Systems 37A05 (Primary) 37A46, 37A50, 05D10 (Secondary) In "IP-sets and polynomial recurrence", Bergelson, Furstenberg, and McCutcheon established the following far reaching extension of Khintchine's recurrence theorem: For any invertible probability preserving system $(X,\mathcal A,μ,T)$, any non-constant polynomial $p\in\mathbb Z[x]$ with $p(0)=0$, any $A\in\mathcal A$, and any $ε>0$, the set $$R_ε^p(A)=\{n\in\mathbb N\,|\,μ(A\cap T^{-p(n)}A)>μ^2(A)-ε\}$$ is IP$^*$, meaning that for any increasing sequence $(n_k)_{k\in\mathbb N}$ in $\mathbb N$, $$\text{FS}((n_k)_{k\in\mathbb N})\cap R_ε^p(A)\neq \emptyset,$$ where $$\text{FS}((n_k)_{k\in\mathbb N})=\{\sum_{j\in F}n_j\,|\,F\subseteq \mathbb N\,\text{ is finite}\text{ and }F\neq\emptyset\}=\{n_{k_1}+\cdots+n_{k_t}\,|\,k_1<\cdots<k_t,\,t\in\mathbb N\}.$$ In view of the potential new applications to combinatorics, this result has led to the question of whether a further strengthening of Khintchine's recurrence theorem holds, namely whether the set $R_ε^p(A)$ is IP$_0^*$ meaning that there exists a $t\in\mathbb N$ such that for any finite sequence $n_1<\cdots<n_t$ in $\mathbb N$, $$\{\sum_{j\in F}n_j\,|\,F\subseteq \{1,...,t\}\text{ and }F\neq \emptyset\}\cap R_ε^p(A)\neq \emptyset.$$ In this paper we give a negative answer to this question by showing that for any given polynomial $p\in\mathbb Z[x]$ with deg$(p)>1$ and $p(0)=0$ there is an invertible probability preserving system $(X,\mathcal A,μ,T)$, a set $A\in\mathcal A$, and an $ε>0$ for which the set $R_ε^p(A)$ is not IP$_0^*$. |
| title | Failure of Khintchine-type results along the polynomial image of IP$_0$ sets |
| topic | Dynamical Systems 37A05 (Primary) 37A46, 37A50, 05D10 (Secondary) |
| url | https://arxiv.org/abs/2307.08764 |