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Antonia W. Bluher

Publications and source records attributed to Antonia W. Bluher.

11 recordsLinked to original sources
arXiv2022

A new identity of Dickson polynomials

A new polynomial identity is found for Dickson polynomials in characteristic 2. The identity is used to prove that the two polynomials $x^{q+1}+x+1/a$ and $C(x)+a$ have the same splitting field over $F$, where $F$ is a field of characteristic 2, $a$ is a nonzero element of $F$, $q=2^n>2$, and $C(x) = x (\sum_{i=0}^{n-1} x^{2^i-1})^{q+1}$ is a Müller--Cohen--Matthews polynomial of degree $(q^2-q)/2$. In addition, a new proof is obtained for the known result that $C(x)$ induces a permutation on $F_{2^m}$ if $2m$ and $n$ are relatively prime.

math.NT↗
arXiv2021

Explicit Artin maps into ${\rm PGL}_2$

Let $G$ be a subgroup of ${\rm PGL}_2({\mathbb F}_q)$, where $q$ is any prime power, and let $Q \in {\mathbb F}_q[x]$ such that ${\mathbb F}_q(x)/{\mathbb F}_q(Q(x))$ is a Galois extension with group $G$. By explicitly computing the Artin map on unramified degree-1 primes in ${\mathbb F}_q(Q)$ for various groups $G$, interesting new results emerge about finite fields, additive polynomials, and conjugacy classes of ${\rm PGL}_2({\mathbb F}_q)$. For example, by taking $G$ to be a unipotent group, one obtains a new characterization for when an additive polynomial splits completely over ${\mathbb F}_q$. When $G = {\rm PGL}_2({\mathbb F}_q)$, one obtains information about conjugacy classes of ${\rm PGL}_2({\mathbb F}_q)$. When $G$ is the group of order 3 generated by $x \mapsto 1 - 1/x$, one obtains a natural tripartite symbol on ${\mathbb F}_q$ with values in ${\mathbb Z}/3{\mathbb Z}$. Some of these results generalize to ${\rm PGL}_2(K)$ for arbitrary fields $K$. Apart from the introduction, this article is written from first principles, with the aim to be accessible to graduate students or advanced undergraduates. An earlier draft of this article was published on the Math arXiv in June 2019 under the title {\it More structure theorems for finite fields}.

math.NT↗
arXiv2021

New Wilson-like theorems arising from Dickson polynomials

Wilson's Theorem states that the product of all nonzero elements of a finite field ${\mathbb F}_q$ is $-1$. In this article, we define some natural subsets $S \subset {\mathbb F}_q^\times$ and find formulas for the product of the elements of $S$, denoted $\prod S$. These new formulas are appealing for the simple, natural description of the sets $S$, and for the simplicity of the product. An example is $\prod\left\{ a \in {\mathbb F}_q^\times : \text{$1-a$ and $3+a$ are nonsquares} \right\} = 2$ if $q \equiv \pm 1 \pmod{12}$, or $-1$ otherwise.

math.NT↗
arXiv2021

Permutation properties of Dickson and Chebyshev polynomials and connections to number theory

The $k$th Dickson polynomial of the first kind, $D_k(x) \in {\mathbb Z}[x]$, is determined by the formula: $D_k(u+1/u) = u^k + 1/u^k$, where $k \ge 0$ and $u$ is an indeterminate. These polynomials are closely related to Chebyshev polynomials and have been widely studied. Leonard Eugene Dickson proved in 1896 that $D_k(x)$ is a permutation polynomial on ${\mathbb F}_{p^n}$, $p$ prime, if and only if GCD$(k,p^{2n}-1)=1$, and his result easily carries over to Chebyshev polynomials when $p$ is odd. This article continues on this theme, as we find special subsets of ${\mathbb F}_{p^n}$ that are stabilized or permuted by Dickson or Chebyshev polynomials. Our analysis also leads to a factorization formula for Dickson and Chebyshev polynomials and some new results in elementary number theory. For example, we show that if $q$ is an odd prime power, then $\prod\left\{ a \in{\mathbb F}_q^\times : \text{$a$ and $4-a$ are nonsquares} \right\} = 2$.

math.NT↗
arXiv2019

A structure theorem for finite fields

We present a new structure theorem for finite fields of odd order that relates multiplicative and additive structure in an interesting way. This theorem has several applications, including an improved understanding of Dickson and Chebyshev polynomials and some formulas with a number-theoretic flavor. This paper is an abridged version of two math arXiv articles by the author [arXiv:1707.06870, arXiv:1707.06877].

math.NT↗
arXiv2015

Gray-coding through nested sets

We consider the following combinatorial question. Let $$ S_0 \subset S_1 \subset S_2 \subset ...\subset S_m $$ be nested sets, where #$(S_i) = i$. A move consists of altering one of the sets $S_i$, $1 \le i \le m-1$, in a manner so that the nested condition still holds and #$(S_i)$ is still $i$. Our goal is to find a sequence of moves that exhausts through all subsets of $S_m$ (other than the initial sets $S_i$) with no repeats. We call this "Gray-coding through nested sets" because of the analogy with Frank Gray's theory of exhausting through integers while altering only one bit at a time. Our main result is an efficient algorithm that solves this problem. As a byproduct, we produce new families of cyclic Gray codes through binary $m$-bit integers.

math.CO↗
arXiv2012

On existence of Budaghyan-Carlet APN hexanomials

Budaghyan and Carlet constructed a family of almost perfect nonlinear (APN) hexanomials over a field with r^2 elements, and with terms of degrees r+1, s+1, rs+1, rs+r, rs+s, and r+s, where r = 2^m and s = 2^n with GCD(m,n)=1. The construction requires a technical condition, which was verified empirically in a finite number of examples. Bracken, Tan, and Tan (arXiv:1110.3177 [cs.it]) proved the condition holds when m = 2 or 4 (mod 6). In this article, we prove that the construction of Budaghyan and Carlet produces APN polynomials for all m and n. In the case where GCD(m,n) = k >= 1, Budaghyan and Carlet showed that the nonzero derivatives of the hexanomials are 2^k-to-one maps from F_{r^2} to F_{r^2}, provided the same technical condition holds. We prove their construction produces hexanomials with this differential property for all m and n.

math.CO↗
arXiv2004

A Swan-like Theorem

Richard G. Swan proved in 1962 that trinomials x^{8k} + x^m + 1 with 8k > m have an even number of irreducible factors, and so cannot be irreducible. In fact, he found the parity of the number of irreducible factors for any square-free trinomial in F_2[x]. We prove a result that is similar in spirit. Namely, suppose n is odd and f(x) = x^n + Sum_{i in S} x^i + 1 in F_2[x], where S subset {i : i odd, i < n/3} Union {i : i = n (mod 4), i < n} We show that if n = +-1 (mod 8) then f(x) has an odd number of irreducible factors, and if n = +=3 (mod 8) then f(x) has an even number of irreducible factors. This has an application to the problem of finding polynomial bases {1,a,a^2,...a^{n-1}} of F_{2^n} such that Tr(a^i) = 0 for all 1 <= i < n.

math.NT↗
arXiv1997

Elementary background in elliptic curves

This paper gives additional background in algebraic geometry as an accompaniment to the article, ``Formal Groups, Elliptic Curves, and some Theorems of Couveignes'' [arXiv:math.NT/9708215]. Section 1 discusses the addition law on elliptic curves, and Sections 2 and 3 explain about function fields, uniformizers, and power series expansions with respect to a uniformizer.

math.NT↗
arXiv1997

Formal groups, elliptic curves, and some theorems of Couveignes

The formal group law of an elliptic curve has seen recent applications to computational algebraic geometry in the work of Couveignes to compute the order of an elliptic curve over finite fields of small characteristic. The purpose of this paper is to explain in an elementary way how to associate a formal group law to an elliptic curve and to expand on some theorems of Couveignes. In addition, the paper serves as background for [J. Number Theory 70 (1998), 127-145]. We treat curves defined over arbitrary fields, including fields of characteristic two or three.

math.NT↗