Past exam of the mathematics course of the University of Cambridge / 2021 / iii / Paper 129 / 4 / c

The Dense Bogolyubov-Ruzsa lemma says that, for every $\alpha >0$, if $S$ has density at least $\alpha$ in a cyclic group of prime order, then $2S-2S$ contains a proper generalized arithmetic progression of rank $O_{\alpha }(1)$ and size ${\Omega }_{\alpha }(|G|)$.

Now suppose $A\subseteq \mathbb{Z}$ and $|A+A|\le K|A|$. The Ruzsa modelling lemma, taken at a sufficiently high fixed Freiman order, supplies $A^{\prime }\subseteq A$ with $|A^{\prime }|\ge |A|/2$ and a Freiman isomorphism from $A^{\prime }$ to a subset $S\subseteq \mathbb{Z}/q\mathbb{Z}$, where $q$ is prime, $q=O_K(|A|)$, and $|S|/q{\gg }_{K}1$. Apply the dense Bogolyubov-Ruzsa lemma to $S$ and transfer the resulting progression back through the Freiman model. We obtain a proper progressionof rank $O_K(1)$ and size $|P_0|{\gg }_K|A|$.

The Plünnecke-Ruzsa inequality givesThere are $|A||P_0|$ pairs $(a,p)$ with $a\in A$ and $p\in P_0$, distributed among the sums in $A+P_0$. Some $x$ therefore has at least $K^{-5}|P_0|{\gg }_K|A|$ representations $x=a+p$. Equivalently,Set $P=x-P_0$. Translation and negation preserve properness and rank. Moreover $P_0\subseteq 2A-2A$ and another use of Plünnecke gives $|P|\le K^4|A|$. Thusas required.