diff --git a/blueprint/src/integers.tex b/blueprint/src/integers.tex new file mode 100644 index 0000000000..c6d3e18b1c --- /dev/null +++ b/blueprint/src/integers.tex @@ -0,0 +1,118 @@ +\chapter{The integer case} + +\begin{theorem}\label{th-ap-int} +There is a constant $c>0$ such that the following holds. Let $\epsilon>0$ and $B,B'\subseteq G$ be regular Bohr sets of rank $d$. Suppose that $A_1\subseteq B$ with density $\alpha_1$ and $A_2$ is such that there exists $x$ with $A_2\subseteq B'-x$ with density $\alpha_2$. Let $S$ be any set with $\abs{S}\leq 2\abs{B}$. There is a regular Bohr set $B''\subseteq B'$ of rank at most +\[d+O_\epsilon(\lo{\alpha_1}^3\lo{\alpha_2})\] +and size +\[\abs{B''}\geq \exp(-O_\epsilon(d\lo{\alpha_1\alpha_2/d}+\lo{\alpha_1}^3\lo{\alpha_2}\lo{\alpha_1\alpha_2/d}))\abs{B'}\] +such that +\[\abs{\langle \mu_{B'}\ast \mu_{A_1}\circ \mu_{A_2},\ind{S}\rangle-\langle \mu_{A_1}\circ \mu_{A_2},\ind{S}\rangle}\leq \epsilon.\] +\end{theorem} +\begin{proof} +\uses{linfty_ap, chang, bohr-size} +To do. +\end{proof} + +\begin{proposition}\label{Lp-orth} +There is a constant $c>0$ such that the following holds. Let $\epsilon >0$ and $p \geq 2$ be an integer. Let $B \subseteq G$ be a regular Bohr set and $A\subseteq B$ with relative density $\alpha$. Let $\nu : G \to \bbr_{\geq 0}$ be supported on $B_\rho$, where $\rho \leq c\epsilon\alpha/\rk(B)$, such that $\norm{\nu}_1=1$ and $\widehat{\nu}\geq 0$. If + \[ \norm{(\mu_A-\mu_B) \circ (\mu_{A}-\mu_B)}_{p(\nu)} \geq \epsilon\, \mu(B)^{-1}, \] + then there exists $p'\ll_\epsilon p$ such that + \[ \norm{ \mu_{A}\circ \mu_{A}}_{p'(\nu)} \geq \left(1+\tfrac{1}{4}\epsilon\right) \mu(B)^{-1}. \] +\end{proposition} +\begin{proof} +\uses{reg-conv, unbalancing} +To do. +\end{proof} + + +\begin{proposition}\label{pos-def-measures} +There is a constant $c>0$ such that the following holds. Let $p \geq 2$ be an even integer. Let $f : G \to \bbr$, let $B \subseteq G$ and $B', B'' \subseteq B_{c/\rk(B)}$ all be regular Bohr sets. Then +\[ \norm{ f\circ f }_{p(\mu_{B'}\circ \mu_{B'}\ast \mu_{B''}\circ \mu_{B''})} \geq \tfrac{1}{2} \norm{f*f}_{p(\mu_B)}. \] +\end{proposition} +\begin{proof} +\uses{bohr-majorise} +To do, +\end{proof} + +\begin{proposition}\label{holder-lifting} +There is a constant $c>0$ such that the following holds. Let $\epsilon >0$. Let $B \subseteq G$ be a regular Bohr set and $A\subseteq B$ with relative density $\alpha$, and let $B' \subseteq B_{c\epsilon\alpha/\rk(B)}$ be a regular Bohr set and $C\subseteq B'$ with relative density $\gamma$. Either +\begin{enumerate} +\item $\abs{ \langle \mu_A*\mu_A, \mu_{C} \rangle - \mu(B)^{-1} } \leq \epsilon \mu(B)^{-1}$ or +\item there is some $p \ll\lo{\gamma}$ such that $\norm{ (\mu_A-\mu_B)*(\mu_A-\mu_B)}_{p(\mu_{B'})} \geq \tfrac{1}{2}\epsilon \mu(B)^{-1}$. +\end{enumerate} +\end{proposition} +\begin{proof} +%\uses{reg-conv} +To do. +\end{proof} + +\begin{proposition}\label{prop-it} +There is a constant $c>0$ such that the following holds. Let $\epsilon>0$ and $p,k\geq 1$ be integers such that $(k,\abs{G})=1$. Let $B,B',B''\subseteq G$ be regular Bohr sets of rank $d$ such that $B''\subseteq B'_{c/d}$ and $A\subseteq B$ with relative density $\alpha$. If + \[ \norm{ \mu_{A}\circ \mu_{A}}_{p(\mu_{k\cdot B'}\circ\mu_{k\cdot B'}\ast \mu_{k\cdot B''}\circ \mu_{k\cdot B''})} \geq \left(1+\epsilon\right) \mu(B)^{-1},\] + then there is a regular Bohr set $B'''\subseteq B''$ of rank at most + \[\rk(B''')\leq d+O_{\epsilon}(\lo{\alpha}^4p^4)\] + and size + \[\abs{B'''}\geq \exp(-O_{\epsilon}(dp\lo{\alpha/d}+\lo{\alpha}^5p^5))\abs{B''}\] + such that + \[ \norm{ \mu_{B'''}*\mu_A }_\infty \geq (1+c\epsilon)\mu(B)^{-1}. \] +\end{proposition} +\begin{proof} +\uses{sift, th-ap-int} +To do. +\end{proof} + +\begin{theorem}\label{flat-on-bohr} +There is a constant $c>0$ such that the following holds. Let ${\epsilon,\delta\in (0,1)}$ and $p,k\geq 1$ be integers such that $(k,\abs{G})=1$. For any $A\subseteq G$ with density $\alpha$ there is a regular Bohr set $B$ with +\[ d=\rk(B) =O_{\epsilon}\left(\lo{\alpha}^5p^4\right) \quad\text{and}\quad \abs{B}\geq \exp\left(-O_{\epsilon,\delta}(\lo{\alpha}^6p^5\lo{\alpha/p})\right)\abs{G} \] +and some $A'\subseteq (A-x)\cap B$ for some $x \in G$ such that +\begin{enumerate} +\item $\abs{A'}\geq (1-\epsilon)\alpha\abs{B}$, +\item $\abs{A'\cap B'}\geq (1-\epsilon)\alpha\abs{B'}$, where $B'=B_{\rho}$ is a regular Bohr set with ${\rho\in (\tfrac{1}{2},1)\cdot c\delta\alpha/d}$, and +\item +\[\norm{\mu_{A'}\circ \mu_{A'}}_{p(\mu_{k\cdot B''}\circ \mu_{k\cdot B''}\ast \mu_{k\cdot B'''}\circ \mu_{k\cdot B'''})} <(1+ \epsilon)\mu(B)^{-1},\] +for any regular Bohr sets $B'' = B'_{\rho'}$ and $B'''=B''_{\rho''}$ satisfying ${\rho',\rho''\in(\frac{1}{2},1)\cdot c\delta\alpha/d}$. +\end{enumerate} +\end{theorem} +\begin{proof} +\uses{prop-it, bourgain-trick, bohr-size} +To do. +\end{proof} + +\begin{theorem}\label{th-int-gen} +There is a constant $c>0$ such that the following holds. Let $\delta,\epsilon\in (0,1)$, let $p \geq 1$ and let $k$ be a positive integer such that $(k,\abs{G})=1$. There is a constant $C=C(\epsilon,\delta,k)>0$ such that the following holds. + +For any finite abelian group $G$ and any subset $A\subseteq G$ with $\abs{A}=\alpha \abs{G}$ there exists a regular Bohr set $B$ with +\[\rk(B)\leq Cp^4\log(2/\alpha)^5\] +and +\[\abs{B}\geq \exp\left(-Cp^5\log(2p/\alpha)\log(2/\alpha)^6\right)\abs{G}\] +and $A' \subseteq (A-x)\cap B$ for some $x\in G$ such that +\begin{enumerate} +\item $\abs{A'}\geq (1-\epsilon)\alpha \abs{B}$, +\item $\abs{A'\cap B'}\geq (1-\epsilon)\alpha\abs{B'}$, where $B'=B_{\rho}$ is a regular Bohr set with $\rho\in (\tfrac{1}{2},1)\cdot c\delta\alpha/dk$, and +\item +\[\norm{(\mu_{A'}-\mu_B)\ast (\mu_{A'}- \mu_B)}_{p(\mu_{k\cdot B'})} \leq \epsilon\frac{\abs{G}}{\abs{B}}.\] +\end{enumerate} +\end{theorem} +\begin{proof} +\uses{Lp-orth, flat-on-bohr, pos-def-measures} +To do. \end{proof} + +\begin{theorem}\label{main-int-count} +If $A\subseteq \{1,\ldots,N\}$ has size $\abs{A}=\alpha N$, then $A$ contains at least +\[\exp(-O(\lo{\alpha}^{12}))N^2\] +many three-term arithmetic progressions. +\end{theorem} +\begin{proof} +\uses{th-int-gen, holder-lifting} +To do. +\end{proof} + +\begin{theorem}\label{3aps-in-ints} +If $A\subseteq \{1,\ldots,N\}$ contains no non-trivial three-term arithmetic progressions then +\[\lvert A\rvert \leq \frac{N}{\exp(-c(\log N)^{1/12})}\] +for some constant $c>0$. +\end{theorem} +\begin{proof} +\uses{main-int-count} +To do. +\end{proof}