A Class of Artin-Schreier Towers with Finite Genus

We study the asymptotic behaviour of the genus in some Artin-Schreier towers of function fields over a finite field, and we present a new class of Artin-Schreier towers having finite genus.     

代数函数域的一些Artin-Schreier型扩张相伴的Riemann-Roch空间

阐明给定代数函数域上一些除子的Riemann-Roch空间是代数几何码构造的基础.给出代数函数域的一些Artin-Schreier型扩张的Riemann-Roch空间的一组基,并应用于编码理论,得到F_(16)上参数分别是[54,43,5],[54,41,7],[54,40,8]的代数几何码.     

Some Artin-Schreier type function fields over finite fields with prescribed genus and number of rational places

We give existence and characterization results for some Artin-Schreier type function fields over finite fields with prescribed genus and number of rational places simultaneously.     

Artin-Schreier extensions in NIP and simple fields

We show that NIP fields have no Artin-Schreier extension, and that simple fields have only a finite number of them.     

Complete arcs arising from a generalization of the Hermitian curve (extended abstract)

We investigate arcs, in projective planes over finite fields, arising from the set of rational points of a generalization of the Hermitian curve. The degree of the arcs is closely related to the number of rational points of a class of Artin-Schreier curves.     

Automorphic forms and the distribution of points on odd-dimensional spheres

We discuss standard pairs of generators of cyclic divisionp-algebras of degreep, and prove forp=3 that any two Artin-Schreier elements are connected by a chain of standard pairs. This result has immediate applications to the presentations of such algebras.     

Universal Hashing and Geometric Codes

We describe a new application of algebraic coding theory to universal hashing and authentication without secrecy. This permits to make use of the hitherto sharpest weapon of coding theory, the construction of codes from algebraic curves. We show in particular how codes derived from Artin-Schreier curves, Hermitian curves and Suzuki curves yield classes of universal hash functions which are substantially better than those known before.     

Note on a theorem of A. Aiba

Recently A. Aiba (J. Number Theory 102 (2003) 118) gave a criterion whether the valuation ring of a wildly ramified Artin-Schreier extension of F_q((T)) is isomorphic to the associated order of this extension. We correct a mistake of (Aiba (2003)) and give easier substitutes for the (modified) condition of that criterion.     

Algebraic theory of differential inequalities

We obtain differential-algebraic analogues of some basic theorems of real algebra and semialgebraic geometry. Proofs are based on: a differential version of the real spectrum of a differential ring containing Q; an Artin-Schreier theory for such rings; the model theory of ordered differential fields. Results include: an algebraic characterization of the differential inequalities which are consequences of a given finite set of algebraic differential equations and inequalities; a differential counterpart of the Hormander-Lojasiewicz inequality.     

On towers of function fields of Artin-Schreier type

In this article we derive strong conditions on the defining equations of asymptotically good Artin-Schreier towers. We will show that at most three kinds of defining equations can give rise to a recursively defined good tower, if we restrict ourselves to prime degrees. ¹A. Garcia and H. Stichtenoth did part of thiswork during their stay at Sabanci University, Istanbul, Turkey (Sept. 2002). ²A. Garcia was partially supported by PRONEX # 662408/1996-3 (CNPq-Brazil).     

The undecidability of pseudo real closed fields

The aim of this note is to establish the following result: THEOREM: Let be a non-empty class of Boolean spaces and letPRC() be the class of pseudo real closed fields whose spaces of orderings belong to . Then the elementary theory ofPRC() is undecidable.Our proof appears to be an interesting application of the theory of Artin-Schreier structures, which has been initiated in [5] for the purpose of characterization of the absolute Galois groups of PRC fields. In Section 1 we define and investigate Frattini covers of Artin-Schreier structures, in analogy with [6], Section 2. In Section 2 we consider the analogues of proofs of [1] and [3], to attain the Theorem.This work corresponds to a part of the doctoral dissertation done under the supervision of Prof. Moshe Jarden at Tel-Aviv University     

Axiomatisations simples des theories des corps de rolle

Using Becker's results we obtain here a simple first order axiomatization, looking like those by Artin-Schreier and also written in the language of fields, for the theory of Rolle fields (i.e. fields with the Rolle's property for every order). In fields having a finite number of orders, we characterize Rolle fields as those which are pythagorean at level 4 and do not admit any algebraic extension of odd degree. Then we give an axiomatization for Rolle fields having exactly 2ⁿ orders (n≥0); in fact, for n=0 we recover an axiomatization of the theory of real-closed fields and for n=1 we get exactly an axiomatization given for the theory of chain-closed fields by the author in [G1]. Finally we prove that a Rolle field with exactly 2ⁿ orders is the intersection of n+1 real closures of the field.      

Orders on computable rings

The Artin-Schreier theorem says that every formally real field has orders. Friedman, Simpson and Smith showed in [6] that the Artin-Schreier theorem is equivalent to ${\mathsf{WKL}}_0$ over ${\mathsf{RCA}}_0$. We first prove that the generalization of the Artin-Schreier theorem to noncommutative rings is equivalent to ${\mathsf{WKL}}_0$ over ${\mathsf{RCA}}_0$. In the theory of orderings on rings, following an idea of Serre, we often show the existence of orders on formally real rings by extending pre-orders to orders, where Zorn's lemma is used. We then prove that “pre-orders on rings not necessarily commutative extend to orders” is equivalent to ${\mathsf{WKL}}_0$.     

Mixed exponential sums over finite fields

In this paper we calculate the conductor of a character that consists of the product of an additive and a multiplicative character. This computation improves the bound for exponential sums given by G. I. Perelmuter. This calculation gives an easy method to compute the conductor associated to a character of the Galois group of the composite of an Artin-Schreier extension and a Kummer extension.

     

On Ordered Division Rings

Prestel introduced a generalization of the notion of an ordering of a field, which is called a semiordering. Prestel's axioms for a semiordered field differ from the usual (Artin-Schreier) postulates in requiring only the closedness of the domain of positivity under x xa ² for nonzero a, instead of requiring that positive elements have a positive product. In this work, this type of ordering is studied in the case of a division ring. It is shown that it actually behaves the same as in the commutative case. Further, it is shown that the bounded subring associated with that ordering is a valuation ring which is preserved under conjugation, so one can associate a natural valuation to a semiordering.     

Déformations formelles des revêtements sauvagement ramifiés de courbes algébriques

In this paper we study formal moduli for wildly ramified Galois covering. We prove a local-global principle. We then focus on the infinitesimal deformations of the Z/p Z-covers. We explicitly compute a deformation of an automorphism of order p which implies a universal obstruction for p>2. By deforming Artin-Schreier equations we obtain a lower bound on the dimension of the local versal deformation ring. At last, by comparing the global versal deformation ring to the complete local ring in a point of a moduli space, we determine the dimensions of the global and local versal deformation rings.

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Déformations formelles des revêtements sauvagement ramifiés de courbes algébriques

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Oblatum 22-II-1999 & 15-XII-1999?Published online: 29 March 2000     

A quasi quadratic time algorithm for hyperelliptic curve point counting

We describe an algorithm to compute the cardinality of Jacobians of ordinary hyperelliptic curves of small genus over finite fields with cost . This algorithm is derived from ideas due to Mestre. More precisely, we state the mathematical background behind Mestre’s algorithm and develop from it a variant with quasi-quadratic time complexity. Among others, we present an algorithm to find roots of a system of generalized Artin-Schreier equations and give results that we obtain with an efficient implementation. Especially, we were able to obtain the cardinality of curves of genus one, two or three in finite fields of huge size. 2000 Mathematics Subject Classification Primary—11S40, 14H42, 11G20, 11G15, 94A60     

Isomorphisms between Artin-Schreier towers

We give a method for efficiently computing isomorphisms between towers of Artin-Schreier extensions over a finite field. We find that isomorphisms between towers of degree over a fixed field can be computed, composed, and inverted in time essentially linear in . The method relies on an approximation process.