Maximal curves from subcovers of the GK-curve

For every $q=n^3$ with $n$ a prime power greater than $2$, the GK-curve is an $\mathbb F_{q^2}$-maximal curve that is not $\mathbb F_{q^2}$-covered by the Hermitian curve. In this paper some Galois subcovers of the GK curve are investigated. We describe explicit equations for some families of quotients of the GK-curve. New values in the spectrum of genera of $\mathbb F_{q^2}$-maximal curves are obtained. Finally, infinitely many further examples of maximal curves that cannot be Galois covered by the Hermitian curve are provided

On the difference between permutation polynomials over finite fields

The well-known Chowla and Zassenhaus conjecture, proven by Cohen in 1990, states that if $p>(d^2-3d+4)^2$, then there is no complete mapping polynomial $f$ in \Fp[x] of degree $d\ge 2$. For arbitrary finite fields \Fq, a similar non-existence result is obtained recently by I\c s\i k, Topuzo\u glu and Winterhof in terms of the Carlitz rank of $f$. Cohen, Mullen and Shiue generalized the Chowla-Zassenhaus-Cohen Theorem significantly in 1995, by considering differences of permutation polynomials. More precisely, they showed that if $f$ and $f+g$ are both permutation polynomials of degree $d\ge 2$ over \Fp, with $p>(d^2-3d+4)^2$, then the degree $k$ of $g$ satisfies $k \geq 3d/5$, unless $g$ is constant. In this article, assuming $f$ and $f+g$ are permutation polynomials in \Fq[x], we give lower bounds for $k$ in terms of the Carlitz rank of $f$ and $q$. Our results generalize the above mentioned result of I\c s\i k et al. We also show for a special class of polynomials $f$ of Carlitz rank $n \geq 1$ that if $f+x^k$ is a permutation of \Fq, with $\gcd(k+1, q-1)=1$, then $k\geq (q-n)/(n+3)$

Cyclotomic function fields over finite fields with irreducible quadratic modulus

Let $\mathbb{F}_q$ be the finite field of order $q$ and $F=\mathbb{F}_q(x)$ the rational function field. In this paper, we give a characterization of the cyclotomic function fields $F(\Lambda_M)$ with modulus $M$, where $M \in \mathbb{F}_q[T]$ is a monic and irreducible polynomial of degree two. We also provide the full automorphism group of $F(\Lambda_M)$ in odd characteristic, extending results of \cite{MXY2016} where the automorphism group of $F(\Lambda_M)$ over $\mathbb{F}_q$ was computed

Locally recoverable codes from automorphism groups of function fields of genus g≥1g \geq 1

A Locally Recoverable Code is a code such that the value of any single coordinate of a codeword can be recovered from the values of a small subset of other coordinates. When we have $\delta$ non overlapping subsets of cardinality $r_i$ that can be used to recover the missing coordinate we say that a linear code $\mathcal{C}$ with length $n$, dimension $k$, minimum distance $d$ has $(r_1,\ldots, r_\delta)$-locality and denote it by $[n, k, d; r_1, r_2,\dots, r_\delta].$ In this paper we provide a new upper bound for the minimum distance of these codes. Working with a finite number of subgroups of cardinality $r_i+1$ of the automorphism group of a function field $\mathcal{F}| \mathbb{F}_q$ of genus $g \geq 1$, we propose a construction of $[n, k, d; r_1, r_2,\dots, r_\delta]$-codes and apply the results to some well known families of function fields

Construction of sequences with high Nonlinear Complexity from the Hermitian Function Field

We provide a sequence with high nonlinear complexity from the Hermitian function field $\mathcal{H}$ over $\mathbb{F}_{q^2}$. This sequence was obtained using a rational function with pole divisor in certain $\ell$ collinear rational places on $\mathcal{H}$, where $2 \leq \ell \leq q$. In particular we improve the lower bounds on the $k$th-order nonlinear complexity obtained by H. Niederreiter and C. Xing; and O. Geil, F. \"Ozbudak and D. Ruano