Counting multiplicative groups with prescribed subgroups

We examine two counting problems that seem very group-theoretic on the surface but, on closer examination, turn out to concern integers with restrictions on their prime factors. First, given an odd prime [Formula: see text] and a finite abelian [Formula: see text]-group [Formula: see text], we consider the set of integers [Formula: see text] such that the Sylow [Formula: see text]-subgroup of the multiplicative group [Formula: see text] is isomorphic to [Formula: see text]. We show that the counting function of this set of integers is asymptotic to [Formula: see text] for explicit constants [Formula: see text] and [Formula: see text] depending on [Formula: see text] and [Formula: see text]. Second, we consider the set of integers [Formula: see text] such that the multiplicative group [Formula: see text] is “maximally non-cyclic”, that is, such that all of its prime-power subgroups are elementary groups. We show that the counting function of this set of integers is asymptotic to [Formula: see text] for an explicit constant [Formula: see text], where [Formula: see text] is Artin’s constant. As it turns out, both of these group-theoretic problems can be reduced to problems of counting integers with restrictions on their prime factors, allowing them to be addressed by classical techniques of analytic number theory.

Values of rational functions in small subgroups of finite fields and the identity testing problem from powers

Motivated by some algorithmic problems, we give lower bounds on the size of the multiplicative groups containing rational function images of low-dimensional affine subspaces of a finite field [Formula: see text] considered as a linear space over a subfield [Formula: see text]. We apply this to the recently introduced algorithmic problem of identity testing of “hidden” polynomials [Formula: see text] and [Formula: see text] over a high degree extension of a finite field, given oracle access to [Formula: see text] and [Formula: see text].