Character sums, primitive elements, and powers in finite fields
Let \(F_{q}\) be a finite field of characteristic \(p\) with \(q=p^{n}\) elements, and \(F_{q^{m}}=F_{q}( \alpha)\) its extension of degree \(m \geq 1\). \textit{H. Davenport} [On primitive roots in finite fields, Q. J. Math., Oxf. Ser. 8, 308-312 (1937; Zbl 0018.10901)] proved that the set \(F_{q}+ \alpha\) contains at least one primitive element of \(F_{q^{m}}\) if \(q\) is sufficiently large with respect to \(m\). The author extends this result to certain subsets of \(F_{q}+ \alpha\) of cardinality at least of the order of magnitude \(O(q^{1/2+ \varepsilon})\). The proof is based on a new bound for incomplete character sums \[ S_{K}( \alpha)= \sum_{k=0}^{K-1} \chi( \alpha+ \zeta_{k}), \qquad 1 \leq K < q^{m}, \] where \( \chi\) is a nontrivial multiplicative character of \(F_{q^{m}}\) and \( \zeta_{0}, \zeta_{1}, \ldots , \zeta_{K-1}\) are elements of \(F_{q^{m}}\) defined as follows: if \(k=k_{1}+k_{2} p+ \cdots +k_{mn} p^{mn-1}\) is the \(p\)-adic expansion of \(0 \leq k < q^{m}\) with \(0 \leq k_{i} < p\), \( \{ \omega_{1}, \ldots , \omega_{n} \}\) is a basis of \(F_{q}\) over \(F_{p}\), and \( \omega_{1}, \ldots , \omega_{mn}\) its completion to a basis of \(F_{q^{m}}\) over \(F_{p}\) then \[ \zeta_{k}=k_{1} \omega_{1}+ \cdots k_{mn} \omega_{mn}. \] The author shows that \[ |S_{K}( \alpha)|< 2.2 (Km)^{1/2} q^{1/4}, \] for \(1 \leq K < q\), and deduces that the set \( \{ \alpha, \alpha+ \zeta_{1}, \ldots, \alpha+ \zeta_{K} \}\), with \( \zeta_{i} \in F_{q}\), contains at least \[ \frac{ \varphi(q^{m}-1)}{q^{m}-1}(K+O(K^{1/2}q^{1/4+ \varepsilon})) \] primitive elements of \(F_{q^{m}}\) for any \( \varepsilon > 0\). Moreover, the author obtains a new upper bound for the longest sequence \[ \alpha, \alpha+ \zeta_{1}, \ldots, \alpha+ \zeta_{L-1} \] of consecutive powers in \(F_{q^{m}}\).
scientific article; zbMATH DE number 1787791
scientific article