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تسجيل مستخدم جديدOptimal lower bounds for multiple recurrence
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Let $(X, mathcal{B},mu,T)$ be an ergodic measure preserving system, $A in mathcal{B}$ and $epsilon>0$. We study the largeness of sets of the form begin{equation*} begin{split} S = left{ ninmathbb{N}colonmu(Acap T^{-f_1(n)}Acap T^{-f_2(n)}Acapldotscap T^{-f_k(n)}A)> mu(A)^{k+1} - epsilon right} end{split} end{equation*} for various families ${f_1,dots,f_k}$ of sequences $f_icolon mathbb{N} to mathbb{N}$. For $k leq 3$ and $f_{i}(n)=i f(n)$, we show that $S$ has positive density if $f(n)=q(p_n)$ where $q in mathbb{Z}[x]$ satisfies $q(1)$ or $q(-1) =0$ and $p_n$ denotes the $n$-th prime; or when $f$ is a certain Hardy field sequence. If $T^q$ is ergodic for some $q in mathbb{N}$, then for all $r in mathbb{Z}$, $S$ is syndetic if $f(n) = qn + r$. For $f_{i}(n)=a_{i}n$, where $a_{i}$ are distinct integers, we show that $S$ can be empty for $kgeq 4$, and for $k = 3$ we found an interesting relation between the largeness of $S$ and the abundance of solutions to certain linear equations in sparse sets of integers. We also provide some partial results when the $f_{i}$ are distinct polynomials.
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