1,720,967 research outputs found
Arithmétique des espaces homogènes de groupes algébriques sur certains corps de fonction
No full textCette thèse étudie des problèmes d'approximation faible et forte pour les espaces homogènes de groupes algébriques définis sur certains corps globaux géométriques de dimension 2 ou des corps de fonctions p-adiques.This dissertation studies weak and strong approximation problems for homogeneous spaces of algebraic groups defined over some two-dimensional geometric global fields, or p-adic function fields
Principe local-global pour les points entiers sur certaines surfaces algébriques
No full textDans cette thèse, nous étudions le problème de l'existence de points entiers sur certaines surfaces algébriques définies sur des corps de nombres, en particulier le corps des nombres rationnels. Dans le premier chapitre, nous introduisons l'historique du problème et quelques progrès récents dans le sujet de notre étude, en particulier les travaux récents de Ghosh–Sarnak, Loughran–Mitankin, et Colliot-Thélène–Wei–Xu. Dans le chapitre 2, nous étudions l'obstruction de Brauer–Manin pour les surfaces cubiques de type Markoff. Nous fournissons d'abord quelques informations sur les variétés de caractères et l'origine naturelle des surfaces cubiques de type Markoff, puis nous calculons explicitement le groupe de Brauer des compactifications lisses et le groupe de Brauer algébrique des surfaces affines. Ensuite, nous utilisons le groupe de Brauer pour prouver l'échec de l'approximation forte qui peut s'expliquer par l'obstruction de Brauer–Manin dans une famille infinie de surfaces, puis donnons des estimations asymptotiques pour la fréquence des obstructions. De plus, nous appliquons la théorie de la réduction, similaire à celle des surfaces de Markoff, dans les travaux récents de Whang pour donner un contre-exemple explicite au principe de Hasse entier pour nos surfaces cubiques de type Markoff. Nous donnons aussi des résultats analogues à ceux sur les surfaces de Markoff à propos de l'obstruction de Brauer–Manin dans quelques cas particuliers de surfaces cubiques de type Markoff. Dans le chapitre 3, nous étudions l'obstruction de Brauer–Manin pour les surfaces de Wehler K3 de type Markoff et suivons la même structure que le chapitre précédent. Nous fournissons d'abord quelques informations sur les surfaces K3 de Wehler et une étude récente des surfaces K3 de type Markoff (MK3), ainsi que les trois familles explicites de surfaces MK3 qui nous intéressent. Ensuite, nous calculons explicitement le groupe de Brauer algébrique des clôtures projectives lisses, puis le groupe de Brauer algébrique des surfaces affines. Ensuite, nous utilisons le groupe de Brauer pour prouver l'échec du principe de Hasse entier qui peut être expliqué par l'obstruction de Brauer–Manin pour trois familles de surfaces MK3, puis donnons quelques estimations asymptotiques pour les échecs de Hasse. Ensuite, nous étudions quelques cas où l'obstruction de Brauer–Manin à l'existence de points entiers et de points rationnels peut disparaître, puis donnons quelques contre-exemples à l'approximation forte qui peut s'expliquer par l'obstruction de Brauer–Manin. De plus, nous donnons quelques exemples explicites qui montrent que des points rationnels existent sur des surfaces MK3 affines. Pour compléter la thèse, dans l'annexe A, nous donnons une brève introduction aux obstructions de descente associées aux torseurs d'Artin–Schreier et à leur relation avec l'obstruction de Brauer–Manin pour les points entiers sur les variétés affines sur un corps de fonctions d'une courbe algébrique sur un corps fini, comme étudiées par Harari et Voloch. Enfin, nous étudions quelques contre-exemples au principe de Hasse entier sur des coniques et des surfaces de Markoff.In this thesis, we study the problem of the existence of integral points on certain algebraic surfaces defined over number fields, particularly the field of rational numbers. In the first chapter, we introduce the history of the problem and some recent progress in the subject of our study, especially the recent work of Ghosh–Sarnak, Loughran–Mitankin, and Colliot-Thélène–Wei–Xu. In Chapter 2, we study the Brauer–Manin obstruction for Markoff-type cubic surfaces. We first provide some background on character varieties and the natural origin of the Markoff-type cubic surfaces, then we explicitly calculate the Brauer group of the smooth compactifications and the algebraic Brauer group of the affine surfaces. Afterward, we use the Brauer group to prove the failure of strong approximation which can be explained by the Brauer–Manin obstruction in an infinite family of surfaces, and then give some counting results for the frequency of the obstructions. Furthermore, we apply the reduction theory, similar to that of Markoff surfaces, in recent work by Whang to give an explicit counterexample to the integral Hasse principle for our Markoff-type cubic surfaces. We also give some analogous results to those on Markoff surfaces about the Brauer–Manin obstruction in some special cases of Markoff-type cubic surfaces. In Chapter 3, we study the Brauer–Manin obstruction for Wehler K3 surfaces of Markoff type and follow the same structure as the previous chapter. We first provide some background on Wehler K3 surfaces and a recent study of the Markoff-type K3 (MK3) surfaces, as well as introduce the three explicit families of MK3 surfaces that interest us. Next, we explicitly calculate the algebraic Brauer group of the smooth projective closures, and then the algebraic Brauer group of the affine surfaces. Afterward, we use the Brauer group to prove the failure of the integral Hasse principle which can be explained by the Brauer–Manin obstruction for three families of MK3 surfaces, and then give some counting results for the Hasse failures. Afterward, we study some cases when the Brauer–Manin obstruction to the existence of integral points and rational points can vanish, then give some counterexamples to strong approximation which can be explained by the Brauer–Manin obstruction. Furthermore, we provide some explicit examples which show that rational points do exist on affine MK3 surfaces. To complete the thesis, in Appendix A, we give a brief introduction to the descent obstructions associated with Artin–Schreier torsors and their relation to the Brauer–Manin obstruction for integral points on affine varieties over global function fields, as studied by Harari and Voloch. Finally, we study some counterexamples to the integral Hasse principle on conics and Markoff surfaces
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Obstructions de Brauer-Manin entières sur les espaces homogènes à stabilisateurs finis nilpotents
Get PDFLe groupe fondamental étale d’un espace homogène d’un groupe algébrique linéaire
No full textInternational audienc
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
- …
