Character-sum bound for primitive normal values of rational functions

Let qq be a prime power, let mm be a positive integer, and let Fqm\mathbb{F}_{q^m} be a finite field with characteristic pp. Write a rational function as

f=f1f2,f=\frac{f_1}{f_2},

where f1,f2Fqm[x]f_1,f_2\in\mathbb{F}_{q^m}[x] are relatively prime, and let n>2n>2 be its degree, with n=n1+n2n=n_1+n_2, where n1n_1 and n2n_2 are the degrees of f1f_1 and f2f_2. Let WW denote the number-of-divisors function and let Ω\Omega denote the number of monic divisors of a polynomial.

Rational-function primitive normal conjecture. There exists an element αFqm\alpha\in\mathbb{F}_{q^m} such that α\alpha and f(α)f(\alpha) are simultaneously primitive normal over Fq\mathbb{F}_q if

qm/2>(2n2)W(qm1)2Ω(xm1)2,q^{m/2}>(2n-2)W(q^m-1)^2\Omega(x^m-1)^2,

provided that f(x)f(x) is not of the form axigh(x)ax^ig^h(x), where ii is an integer, 1hqm11\neq h\mid q^m-1, and aFqma\in\mathbb{F}_{q^m}^*, and, if n1n2n_1\neq n_2, then pn2p\nmid n_2.

This is presented as an extension of earlier theorems and is based on computational experiments with related rational forms. Establishing the conjectured sufficient condition requires further analysis of the behavior of primitive normal elements over finite fields.

Sources & referencesView supporting material

Primary source

Himangshu Hazarika, Dhiren Kumar Basnet and Giorgos Kapetanakis, “On existence of primitive normal elements of rational form over finite fields of even characteristic”, arXiv:2005.01216 (2024).

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