Additive and Multiplicative Structure in Matrix Spaces.

Let $A$ be a set of $N$ matrices. Let $g(A)\ccolon=|A+A|+|A\cdot A|$, where $A+A=\{a_1 + a_2 \mid a_i \in A\}$ and $A\cdot A=\{a_1a_2 \mid a_i \in A\}$ are the sum set and product set. We prove that if the determinant of the difference of any two distinct matrices in $A$ is nonzero, then $g(A)$ cannot be bounded below by $cN$ for any constant $c$. We also prove that if $A$ is a set of $d\times d$ symmetric matrices, then there exists $\varepsilon=\varepsilon (d)>0$ such that $g(A)>N^{1+\varepsilon}.$ For the first result, we use the bound on the number of factorizations in a generalized progression. For the symmetric case, we use a technical proposition which provides an affine space $V$ containing a large subset $E$ of $A$, with the property that if an algebraic property holds for a large subset of $E$, then it holds for $V$. Then we show that the system $\{a^2\ccolon a\in V\}$ is commutative, allowing us to decompose ${\mathbb R}^d$ as eigenspaces simultaneously, so we can finish the proof with induction and a variant of the Erdős–Szemerédi argument. [ABSTRACT FROM AUTHOR]