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On products and shifted products of residues modulo \(p\). (English) Zbl 1168.11009

It is shown that if \(p\) is prime and \(\mathcal A, \mathcal B, \mathcal C, \mathcal D\) are large subsets of \(\mathbb Z_p\), then the equation \(ab+1=cd, a\in \mathcal A, b\in \mathcal B, c\in \mathcal C, d\in \mathcal D\) has a solution. This is a multiplicative analogue of the author’s earlier result where it was shown that the equation \(a+b=cd\) has a solution. Several interesting known and new results are shown to follow as a consequence, such as the solvability of both Legendre symbol equations \((\frac{a+b}{p})=1\) and \((\frac{a'+b'}{p})=-1\), where \(a'\in \mathcal A, b'\in \mathcal B\). The proof uses simple combinatorial arguments and summation techniques.

MSC:

11D45 Counting solutions of Diophantine equations
11B75 Other combinatorial number theory
11L40 Estimates on character sums
11A07 Congruences; primitive roots; residue systems