Abstract:
We find the nearly optimal size of a set $A\subset [N] := \{1,\dots ,N\}$ so that the product set $AA$ satisfies either (i) $|AA| \sim |A|^2/2$ or (ii) $|AA| \sim |[N][N]|$. This settles problems recently posed in a paper of J. Cilleruelo, D. S. Ramana and O. Ramaré.
The research of the author was supported in part by the individual NSF grant DMS-1501982. Some of this work was carried out at MSRI, Berkeley, during the Spring semester of 2017, partially supported by NSF grant DMS-1440140.
Citation:
K. Ford, “Extremal properties of product sets”, Harmonic analysis, approximation theory, and number theory, Collected papers. Dedicated to Academician Sergei Vladimirovich Konyagin on the occasion of his 60th birthday, Trudy Mat. Inst. Steklova, 303, MAIK Nauka/Interperiodica, Moscow, 2018, 239–245; Proc. Steklov Inst. Math., 303 (2018), 220–226