Dado um semigrupo S e a topologia discreta sobre ele, é possível estender a operação ao compactificado de Stone-Cech beta(S) de forma que seja contínua à direita. Diversas propriedades algébricas tais como cancelatividade, comutatividade e ser grupo implicam em propriedades algébrico-topológicas de beta(S). Em particular, o conjunto dos naturais com a soma e/ou o produto é o mais explorado: resultados tais como a existência de 2^c ideais á esquerda minimais e de cadeias decrescentes de idempotentes são mostrados e suas consequências discutidas. / Given a semigroup S and its discrete topology, it is possible to extend its operation to its Stone-Cech compactification beta(S) so that it is right-continuous. Several algebraic properties such as cancellativity, commutativity annd being a group influence topological-algebraic properties of beta(S). Most especially, the set of natural numbers with addition and/or multiplication is explored: results such as the existence of 2^c minimal left ideals or of decreasing chains of idempotents are shown and their consequences analysed.

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In this thesis, we develop a relational localisation theory for topological algebras, i.e., a theory that studies local approximations of a topological algebra’s relational counterpart. In order to provide an appropriate framework for our considerations, we first introduce a general Galois theory between continuous functions and closed relations on an arbitrary topological space. Subsequently to this rather foundational discussion, we establish the desired localisation theory comprising the identification of suitable subsets, the description of local structures, and the retrieval of global information from local data. Among other results, we show that the restriction process with respect to a sufficiently large collection of local approximations of a Hausdorff topological algebra extends to a categorical equivalence between the topological quasivariety generated by the examined structure and the one generated by its localisation. Furthermore, we present methods for exploring topological algebras possessing certain operational compactness properties. Finally, we explain the developed concepts and obtained results in the particular context of three important classes of topological algebras by analysing the local structure of essentially unary topological algebras, topological lattices, and topological modules of compact Hausdorff topological rings.

In this thesis, we develop a relational localisation theory for topological algebras, i.e., a theory that studies local approximations of a topological algebra’s relational counterpart. In order to provide an appropriate framework for our considerations, we first introduce a general Galois theory between continuous functions and closed relations on an arbitrary topological space. Subsequently to this rather foundational discussion, we establish the desired localisation theory comprising the identification of suitable subsets, the description of local structures, and the retrieval of global information from local data. Among other results, we show that the restriction process with respect to a sufficiently large collection of local approximations of a Hausdorff topological algebra extends to a categorical equivalence between the topological quasivariety generated by the examined structure and the one generated by its localisation. Furthermore, we present methods for exploring topological algebras possessing certain operational compactness properties. Finally, we explain the developed concepts and obtained results in the particular context of three important classes of topological algebras by analysing the local structure of essentially unary topological algebras, topological lattices, and topological modules of compact Hausdorff topological rings.