Making computers automatically find the appropriate meaning of words in context is an interesting problem that has proven to be one of the most challenging tasks in natural language processing (NLP). Widespread potential applications of a possible solution to the problem could be envisaged in several NLP tasks such as text simplification, language learning, machine translation, query expansion, information retrieval and text summarization. Ambiguity of words has always been a challenge in these applications, and the traditional endeavor to solve the problem of this ambiguity, namely doing word sense disambiguation using resources like WordNet, has been fraught with debate about the feasibility of the granularity that exists in WordNet senses. The recent trend has therefore been to move away from enforcing any given lexical resource upon automated systems from which to pick potential candidate senses,and to instead encourage them to pick and choose their own resources. Given a sentence with a target ambiguous word, an alternative solution consists of picking potential candidate substitutes for the target, filtering the list of the candidates to a much shorter list using various heuristics, and trying to match these system predictions against a human generated gold standard, with a view to ensuring that the meaning of the sentence does not change after the substitutions. This solution has manifested itself in the SemEval 2007 task of lexical substitution and the more recent SemEval 2010 task of cross-lingual lexical substitution (which I helped organize), where given an English context and a target word within that context, the systems are required to provide between one and ten appropriate substitutes (in English) or translations (in Spanish) for the target word. In this dissertation, I present a comprehensive overview of state-of-the-art research and describe new experiments to tackle the tasks of lexical substitution and cross-lingual lexical substitution. In particular I attempt to answer some research questions pertinent to the tasks, mostly focusing on completely unsupervised approaches. I present a new framework for unsupervised lexical substitution using graphs and centrality algorithms. An additional novelty in this approach is the use of directional similarity rather than the traditional, symmetric word similarity. Additionally, the thesis also explores the extension of the monolingual framework into a cross-lingual one, and examines how well this cross-lingual framework can work for the monolingual lexical substitution and cross-lingual lexical substitution tasks. A comprehensive set of comparative investigations are presented amongst supervised and unsupervised methods, several graph based methods, and the use of monolingual and multilingual information.

This thesis deals with automatic acquiring thesauri from Wikipedia. It describes Wikipedia as a suitable data set for thesauri acquiring and also methods for computing semantic similarity of terms are described. The thesis also contains a description of concepts and implementation of the system for automatic thesauri acquiring. Finally, the implemented system is evaluated by the standard metrics, such as precision or recall.

Dans le parler des jeunes du Mexique, on retrouve, entre autres formes de vitalité linguistique, un usage fréquent de jeux phoniques qui transforment ou substituent des lexèmes et syntagmes sans qu'il y ait altération significative du signifié. Ce phénomène linguistique utilise trois procédés phoniques: la suffixation parasitaire, la substitution lexicale basée sur la similitude phonique et l'enchaînement en écho. Le présent mémoire contribue à remplir un vide dans la littérature scientifique et faire connaître ce phénomène peu étudié en explorant les aspects formels linguistiques et les fonctions sociales. Nous cherchons à établir les relations qui existent entre la structure linguistique et les fonctions linguistiques et sociales inhérentes à l'usage de jeux phoniques. L'étude empirique menée dans la ville de Puebla (Mexique) a permis la collecte d'un corpus de plus de deux cents jeux phoniques et la réalisation d'entrevues sociolinguistiques avec dix-huit jeunes de Puebla usagers des expressions étudiées. Nous proposons une classification du corpus de jeux phoniques basée sur la structure formelle afin d'apprécier que le phénomène réponde à des patrons fixes de construction. Suite à une analyse thématique des entrevues que nous appuyons d'exemples de jeux phoniques, nous verrons que l'usage de jeux phoniques joue principalement une fonction poétique, ludique ainsi qu'une fonction de cohésion sociale et une fonction identitaire. La présente étude confirme que le caractère non normatif et cryptique intrinsèque aux jeux de mots sert à renforcer ces fonctions sociales, à la manière d'un parler argotique. / In the youth speech of Mexico a frequent use of phonic wordplays that transform or substitute lexemes and phrases without significant alteration of the signifier are found among linguistic vitality signs. This linguistic phenomenon uses three phonic techniques: parasitic suffixation, lexical substitution based on phonic similarity and echo chaining. This report contributes by filling a void in the scientific literature and to expose this overlooked phenomenon by exploring its formal linguistic aspects and social functions. The aim of this study is to establish the relationship between linguistic structure and linguistic and social functions inherent to phonic wordplay use. The empiric study conducted in Puebla (Mexico) allowed the collection of a corpus of more than two hundred phonic wordplays and the realization of sociolinguistic interviews with eighteen Puebla youth, who employ the studied expressions. This work proposes a classification of the corpus based on their formal structure in order to highlight that the phenomenon conforms to fixed patterns of construction. After a thematic analysis of the interviews, with an emphasis on phonic wordplay examples, we observe that the use of phonic wordplays mainly accomplishes poetic and ludic, as well as social cohesion and identity functions. This study confirms that the non-normative and cryptic characteristics intrinsic to phonic wordplays help to reinforce the aforementioned social functions as an argotic speech. / En el habla de los jóvenes de México encontramos, entre otras muestras de vitalidad lingüística, un uso frecuente de juegos fónicos que transforman o sustituyen lexemas y sintagmas sin que haya alteración significativa del significado. Este fenómeno lingüístico utiliza tres recursos fónicos: la sufijación parasitaria, la sustitución léxica por similitud fónica y el encadenamiento en eco. La presente memoria contribuye a llenar un vacío en la literatura científica y dar a conocer este fenómeno poco estudiado, explorando los aspectos formales lingüísticos y las funciones sociales. Buscamos establecer las relaciones que existen entre la estructura lingüística y las funciones lingüísticas y sociales inherentes al uso de los juegos fónicos. El estudio empírico que se llevó a cabo en la ciudad de Puebla (México) permitió la recolección de un corpus de más de doscientos juegos fónicos y la realización de entrevistas sociolingüísticas con dieciocho jóvenes poblanos usuarios de las expresiones estudiadas. Proponemos una clasificación del corpus de juegos fónicos basada en la estructura formal para apreciar que el fenómeno responde a patrones fijos de construcción. Tras un análisis temático de las entrevistas, que apoyamos con ejemplos de juegos fónicos, veremos que el uso de juegos fónicos cumple principalmente una función poética, lúdica, así como una función de cohesión social y una función identitaria. El presente estudio confirma que el carácter no normativo y críptico intrínseco a los juegos fónicos sirve para reforzar estas funciones sociales, a modo de habla argótica.