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Published online by Cambridge University Press: 08 September 2023
In our paper, we study multiplicative properties of difference sets $A-A$ for large sets
$A \subseteq {\mathbb {Z}}/q{\mathbb {Z}}$ in the case of composite q. We obtain a quantitative version of a result of A. Fish about the structure of the product sets
$(A-A)(A-A)$. Also, we show that the multiplicative covering number of any difference set is always small.