A question related to sum-product


Above is a pdf file about an question related to the Sum-product phenomenon introduced by Erdos and Szemeredi.

Fix A \subset \mathbb{R}^+ to be finite and nonempty. We are interested in nontrivial lower bounds for |A + A \cdot A|. The best known bounds in such generality come from the Szemeredi-Trotter theorem from incidence geometry giving |A + A \cdot A| \gg |A|^{3/2}. Though when one restricts to A \subset \mathbb{Z}^+, one has |A+A \cdot A| \geq |A|^2 + |A| -1. We give a short proof of this fact in the above file.

Recently, Oliver Roche-Newton, Imre Ruzsa, Chen-Yen Shen and Ilya Shkredov have made progress! They show both that the previously known upper and lower bounds are not sharp. It is still certainly plausible that for any $\epsilon > 0$, |AA+A| \gg_{\epsilon} |A|^{2- \epsilon}.

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