An Application of the Gauss Lemma to the Study of Pseudorandom Sequences Based on Quadratic Residues

In the context of the study of pseudorandom sequences that use quadratic residues modulo the prime $p$, the constructive description of the set of prime moduli for which given integers are quadratic residues is considered. Using the Gauss Lemma, we prove a criterion of combinatorial nature for a given integer $a$ to be a quadratic residue prime modulo $p$. It is shown how to apply this criterion to the problem of effective description of the prime moduli $p$ satisfying the equation $\bigl(\frac ap\bigr)=1$ for each $p$ from a given finite set $M$.