Une nouvelle borne supérieure sur le cardinal des codes de sous-espaces d'un espace vectoriel fini est établie grâce à la méthode de la programmation semidéfinie positive. Ces codes sont d'intérêt dans le cadre du codage de réseau (network coding). Ensuite, par la même méthode, l'on démontre une borne sur le cardinal des ensembles qui évitent une distance donnée dans l'espace de Johnson et qui est obtenue par une variante d'un programme de Schrijver. Les résultats numériques permettent d'améliorer les bornes existantes sur le nombre chromatique mesurable de l'espace Euclidien. Une hiérarchie de programmes semidéfinis positifs est construite à partir de certaines matrices issues des complexes simpliciaux. Ces programmes permettent d'obtenir une borne supérieure sur le nombre d'indépendance d'un graphe. Aussi, cette hiérarchie partage certaines propriétés importantes avec d'autres hiérarchies classiques. A titre d'exemple, le problème de déterminer le nombre d'indépendance des graphes de Paley est analysé. / We apply the semidefinite programming method to obtain a new upper bound on the cardinality of codes made of subspaces of a linear vector space over a finite field. Such codes are of interest in network coding.Next, with the same method, we prove an upper bound on the cardinality of sets avoiding one distance in the Johnson space, which is essentially Schrijver semidefinite program. This bound is used to improve existing results on the measurable chromatic number of the Euclidean space.We build a new hierarchy of semidefinite programs whose optimal values give upper bounds on the independence number of a graph. This hierarchy is based on matrices arising from simplicial complexes. We show some properties that our hierarchy shares with other classical ones. As an example, we show its application to the problem of determining the independence number of Paley graphs.

We apply the semidefinite programming method to obtain a new upper bound on the cardinality of codes made of subspaces of a linear vector space over a finite field. Such codes are of interest in network coding.Next, with the same method, we prove an upper bound on the cardinality of sets avoiding one distance in the Johnson space, which is essentially Schrijver semidefinite program. This bound is used to improve existing results on the measurable chromatic number of the Euclidean space.We build a new hierarchy of semidefinite programs whose optimal values give upper bounds on the independence number of a graph. This hierarchy is based on matrices arising from simplicial complexes. We show some properties that our hierarchy shares with other classical ones. As an example, we show its application to the problem of determining the independence number of Paley graphs.

CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Este trabalho tem como objetivo estudar os parametros de um codigo projetivo gerado por um conjunto algebrico torico X que e parametrizado por uma quantidade nita de monomios em varias variaveis. Tambem podemos obter conjuntos algebricos toricos associados a matrizes de incidencia de grafos e cluters, e nestes casos obtemos resultados mais precisos, ja que os conjuntos algebricos toricos obtidos sao parametrizados por monomios com mesmo grau. Nos capitulos iniciais sao apresentados os conceitos basicos que servirao de ferramentas para atingir estes objetivos. / This work aims at studying the parameters of a projective code generated by an algebraic toric set X which is parameterized by a nite number of monomials in several variables. We also can obtain algebraic toric sets associated to graph or clutter incidence matrices and in these cases we obtain more precise results since the algebraic toric sets which are obtained are parameterized by monomials of the same degree. In the rst chapters we introduce basic concepts which will serve as tools to reach our aim. / Dissertação (Mestrado)