arXiv subjects

Massimo Giulietti

Publications and source records attributed to Massimo Giulietti.

At least 37 records · Page 2Linked to original sources

arXiv2015

On maximal curves that are not quotients of the Hermitian curve

For each prime power $\ell$ the plane curve $\mathcal X_\ell$ with equation $Y^{\ell^2-\ell+1}=X^{\ell^2}-X$ is maximal over $\mathbb{F}_{\ell^6}$. Garcia and Stichtenoth in 2006 proved that $\mathcal X_3$ is not Galois covered by the Hermitian curve and raised the same question for $\mathcal X_\ell$ with $\ell>3$; in this paper we show that $\mathcal X_\ell$ is not Galois covered by the Hermitian curve for any $\ell>3$. Analogously, Duursma and Mak proved that the generalized GK curve $\mathcal C_{\ell^n}$ over $\mathbb{F}_{\ell^{2n}}$ is not a quotient of the Hermitian curve for $\ell>2$ and $n\ge 5$, leaving the case $\ell=2$ open; here we show that $\mathcal C_{2^n}$ is not Galois covered by the Hermitian curve over $\mathbb{F}_{2^{2n}}$ for $n\geq5$.

math.AG↗

arXiv2015

Constant dimension codes from Riemann-Roch spaces

Some families of constant dimension codes arising from Riemann-Roch spaces associated to particular divisors of a curve $\X$ are constructed. These families are generalizations of the one constructed by Hansen

cs.IT↗

arXiv2015

Large $p$-groups of automorphisms of algebraic curves in characteristic $p$

Let $S$ be a $p$-subgroup of the $\mathbb K$-automorphism group $Aut(\mathcal X)$ of an algebraic curve $\mathcal X$ of genus $g\ge 2$ and $p$-rank $γ$ defined over an algebraically closed field $\mathbb{K}$ of characteristic $p\geq 3$. Nakajima proved that if $γ\ge 2$ then $|S|\leq \textstyle\frac{p}{p-2}(g-1)$. If equality holds, $\mathcal X$ is a Nakajima extremal curve. We prove that if $$|S|>\textstyle\frac{p^2}{p^2-p-1}(g-1)$$ then one of the following cases occurs: (i) $γ=0$ and the extension $\mathbb K(\mathcal X)|\mathbb K(\mathcal X)^S$ completely ramifies at a unique place, and does not ramify elsewhere. (ii) $|S|=p$, and $\mathcal X$ is an ordinary curve of genus $g=p-1$. (iii) $\mathcal X$ is an ordinary, Nakajima extremal curve, and $\mathbb K(\mathcal X)$ is an unramified Galois extension of a function field of a curve given in (ii). There are exactly $p-1$ such Galois extensions. Moreover, if some of them is an abelian extension then $S$ has maximal nilpotency class. The full $\mathbb{K}$-automorphism group of any Nakajima extremal curve is determined, and several infinite families of Nakajima extremal curves are constructed by using their pro-$p$ fundamental groups.

math.AG↗

arXiv2015

Further results on multiple coverings of the farthest-off points

Multiple coverings of the farthest-off points ($(R,μ)$-MCF codes) and the corresponding $(ρ,μ)$-saturating sets in projective spaces $PG(N,q)$ are considered. We propose and develop some methods which allow us to obtain new small $(1,μ)$-saturating sets and short $(2,μ)$-MCF codes with $μ$-density either equal to 1 (optimal saturating sets and almost perfect MCF-codes) or close to 1 (roughly $1+1/cq$, $c\ge1$). In particular, we provide new algebraic constructions and some bounds. Also, we classify minimal and optimal $(1,μ)$-saturating sets in $PG(2,q)$, $q$ small.

math.CO↗

arXiv2015

Unitary graphs and classification of a family of symmetric graphs with complete quotients

A finite graph $Γ$ is called $G$-symmetric if $G$ is a group of automorphisms of $Γ$ which is transitive on the set of ordered pairs of adjacent vertices of $Γ$. We study a family of symmetric graphs, called the unitary graphs, whose vertices are flags of the Hermitian unital and whose adjacency relations are determined by certain elements of the underlying finite fields. Such graphs admit the unitary groups as groups of automorphisms, and they play a significant role in the classification of a family of symmetric graphs with complete quotients such that an associated incidence structure is a doubly point-transitive linear space. We give this classification in the paper and also investigate combinatorial properties of the unitary graphs.

math.CO↗

arXiv2015

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.

math.AG↗

arXiv2014

Tables of parameters of symmetric configurations $v_{k}$

Tables of the currently known parameters of symmetric configurations are given. Formulas for parameters of the known infinite families of symmetric configurations are presented as well. The results of the recent paper [18] are used. This work can be viewed as an appendix to [18], in the sense that the tables given here cover a much larger set of parameters.

math.CO↗

arXiv2013

Large $3$-groups of automorphisms of algebraic curves in characteristic $3$

Let $S$ be a $p$-subgroup of the $\K$-automorphism group $\aut(\cX)$ of an algebraic curve $\cX$ of genus $\gg\ge 2$ and $p$-rank $γ$ defined over an algebraically closed field $\mathbb{K}$ of characteristic $p\geq 3$.In this paper we prove that if $|S|>2(\gg-1)$ then one of the following cases occurs. \begin{itemize} \item[(i)] $γ=0$ and the extension $\K(\cX)/\K(\cX)^S$ completely ramifies at a unique place, and does not ramify elsewhere. \item[(ii)] $γ>0$, $p=3$, $\cX$ is a general curve, $S$ attains the Nakajima's upper bound $3(γ-1)$ and $\K(\cX)$ is an unramified Galois extension of the function field of a general curve of genus $2$ with equation $Y^2=cX^6+X^4+X^2+1$ where $c\in\K^*$. \end{itemize} Case (i) was investigated by Stichtenoth, Lehr, Matignon, and Rocher.

math.AG↗

arXiv2013

Complete arcs and complete caps from cubics with an isolated double point

Small complete arcs and caps in Galois spaces over finite fields $\fq$ with characteristic greater than 3 are constructed from cubic curves with an isolated double point. For $m$ a divisor of $q+1$, complete plane arcs of size approximately $q/m$ are obtained, provided that $(m,6)=1$ and $m<\{1}{4}q^{1/4}$. If in addition $m=m_1m_2$ with $(m_1,m_2)=1$, then complete caps of size approximately $\{m_1+m_2}{m}q^{N/2}$ in affine spaces of dimension $N\equiv 0 \pmod 4$ are constructed.

math.CO↗

arXiv2013

Small complete caps from nodal cubics

Bicovering arcs in Galois affine planes of odd order are a powerful tool for constructing complete caps in spaces of higher dimensions. In this paper we investigate whether some arcs contained in nodal cubic curves are bicovering. For $m_1$, $m_2$ coprime divisors of $q-1$, bicovering arcs in $AG(2,q)$ of size $k\le (q-1)\frac{m_1+m_2}{m_1m_2}$ are obtained, provided that $(m_1m_2,6)=1$ and $m_1m_2<\sqrt[4]{q}/3.5$. Such arcs produce complete caps of size $kq^{(N-2)/2}$ in affine spaces of dimension $N\equiv 0 \pmod 4$. For infinitely many $q$'s these caps are the smallest known complete caps in $AG(N,q)$, $N \equiv 0 \pmod 4$.

math.CO↗

arXiv2012

On constructions and parameters of symmetric configurations v_{k}

The spectrum of possible parameters of symmetric configurations is investigated. We both survey known constructions and results, and propose some new construction methods. Many new parameters are obtained, in particular for cyclic symmetric configurations, which are equivalent to deficient cyclic difference sets. Both Golomb rulers and modular Golomb rulers are a key tool in our investigation. Several new upper bounds on the minimum integer E(k) such that for each v >= E(k) there exists a symmetric configuration v_{k} are obtained. Upper bounds of the same type are provided for cyclic symmetric configurations. From the standpoint of applications, it should be noted that our results extend the range of possible parameters of LDPC codes, generalized LDPC codes, and quasi-cyclic LDPC codes.

math.CO↗

arXiv2011

Transitive A_6-invariant k-arcs in PG(2,q)

For $q=p^r$ with a prime $p\ge 7$ such that $q \equiv 1$ or $19\pmod {30},$ the desarguesian projective plane $PG(2,q)$ of order $q$ has a unique conjugacy class of projectivity groups isomorphic to the alternating group $A_6$ of degree 6. For a projectivity group $Γ\cong A_6$ of $PG(2,q)$, we investigate the geometric properties of the (unique) $Γ$-orbit $\mathcal{O}$ of size 90 such that the 1-point stabilizer of $Γ$ in $\mathcal O$ is a cyclic group of order 4. Here $\mathcal O$ lies either in $PG(2,q)$ or in $PG(2,q^2)$ according as 3 is a square or a non-square element in $GF(q)$. We show that if $q\geq 349$ and $q\neq 421$, then $\mathcal O$ is a 90-arc, which turns out to be complete for $q=349, 409, 529, 601,661.$ Interestingly, $\mathcal O$ is the smallest known complete arc in $PG(2,601)$ and in $PG(2,661).$ Computations are carried out by MAGMA.

math.CO↗

arXiv2011

Large 2-groups of automorphisms of curves with positive 2-rank

Let K be an algebraically closed field of characteristic 2, and let X be a curve over K of genus g>1 and 2-rank r>0. For 2-subgroups S of the K-automorphism group Aut(X) of X, the Nakajima bound is |S| < 4g-3. For every g=2^h+1>8, we construct a curve X attaining the Nakajima bound and determine its relevant properties: X is a bielliptic curve with r=g, and its K-automorphism group has a dihedral K-automorphism group of order 4(g-1) which fixes no point in X. Moreover, we provide a classification of 2-groups S of K-automorphisms not fixing a point of X and such that |S|> 2g-1.

math.AG↗

arXiv2009

Some combinatorial aspects of constructing bipartite-graph codes

We propose geometrical methods for constructing square 01-matrices with the same number n of units in every row and column, and such that any two rows of the matrix contain at most one unit in common. These matrices are equivalent to n-regular bipartite graphs without 4-cycles, and therefore can be used for the construction of efficient bipartite-graph codes such that both the classes of its vertices are associated with local constraints. We significantly extend the region of parameters m,n for which there exist an n-regular bipartite graph with 2m vertices and without 4-cycles. In that way we essentially increase the region of lengths and rates of the corresponding bipartite-graph codes. Many new matrices are either circulant or consist of circulant submatrices: this provides code parity-check matrices consisting of circulant submatrices, and hence quasi-cyclic bipartite-graph codes with simple implementation.

math.CO↗

arXiv2009

Quotient curves of the GK curve

For every $q=l^3$ with $l$ a prime power greater than 2, the GK curve $X$ is an $F_{q^2}$-maximal curve that is not $F_{q^2}$-covered by any $F_{q^2}$-maximal Deligne-Lusztig curve. Interestingly, $X$ has a very large $F_{q^2}$-automorphism group with respect to its genus. In this paper we compute the genera of a large variety of curves that are Galois-covered by the GK curve, thus providing several new values in the spectrum of genera of $F_{q^2}$-maximal curves.

math.AG↗

arXiv2009

AG codes on certain maximal curves

Algebraic Geometric codes associated to a recently discovered class of maximal curves are investigated. As a result, some linear codes with better parameters with respect to the previously known ones are discovered, and 70 improvements on MinT's tables are obtained.

math.AG↗

arXiv2009

On some open problems on maximal curves

In this paper we solve three open problems on maximal curves with Frobenius dimension 3. In particular, we prove the existence of a maximal curve with order sequence (0,1,3,q).

math.AG↗

arXiv2009

Linear nonbinary covering codes and saturating sets in projective spaces

Let A_{R,q} denote a family of covering codes, in which the covering radius R and the size q of the underlying Galois field are fixed, while the code length tends to infinity. In this paper, infinite sets of families A_{R,q}, where R is fixed but q ranges over an infinite set of prime powers are considered, and the dependence on q of the asymptotic covering densities of A_{R,q} is investigated. It turns out that for the upper limit of the covering density of A_{R,q}, the best possibility is O(q). The main achievement of the present paper is the construction of asymptotic optimal infinite sets of families A_{R,q} for any covering radius R >= 2. We first showed that for a given R, to obtain optimal infinite sets of families it is enough to construct R infinite families A_{R,q}^{(0)},A_{R,q}^{(1)},...,A_{R,q}^{(R-1)} such that, for all u >= u_{0}, the family A_{R,q}^{(v)} contains codes of codimension r_{u}=Ru+v and length f_{q}^{v}(r_{u}) where f_{q}^{v}(r)=O(q^{(r-R)/R) and u_{0} is a constant. Then, we were able to construct the needed families A_{R,q}^{(v)} for any covering radius R >= 2, with q ranging over the (infinite) set of R-th powers. For each of these families A_{R,q}^{(v)}, the lower limit of the covering density is bounded from above by a constant independent of q.

math.CO↗