The algebra of densities can be seen to have origins dating back to the 19th century where densities were used to find invariants of the modular group. Since then they have continued to be a source of projective invariants and cocycles related with the projective group, most notably the Schwarzian derivative. One of the first times that the algebra of densities appears in the literature in a similar guise to the way we shall introduce it, is in the work of T.Y. Thomas. He showed that a projective connection on a manifold allows one to determine a canonical affine connection on the total space of a certain bundle which is now known as Thomas' bundle. More recently they have appeared, with the definition we shall use, by H. Khudaverdian and Th. Voronov when studying second order operators generating certain brackets. Of prime importance in this situation is the case of Gerstenhaber algebras and in particular the Batalin-Vilkovisky operator on the odd cotangent bundle. They have also been used by V.Y. Ovsienko and his group in the area of equivariant quantization which is a topic we shall come across in the text. Densities also regularly appear in physics. For example the correct interpretation of a wavefunction is a half-density on a manifold, and this explains their transformation properties under the Galilean group. These results motivate a study into the geometric structure of the algebra of densities as an object in their own right. We shall see that by considering them as a whole algebra many classical results have a clear geometrical picture. Moreover one finds that there are a wealth of areas within this algebra still to explore. We focused on two fundamental classes of objects, differential operators and Poisson structures. The results we find lead to interesting formula for certain equivariantly defined differential operators which can be applied to gain a wide class of cocycles similar to the Schwarzian derivative. We also find very intimate links with Batalin-Vilkovisky geometry and the methods we use show that it may be useful to consider the full algebra of densities when entering into this arena.

Loop homotopy Lie algebras, which appear in closed string field theory, are a generalization of homotopy Lie algebras. For a loop homotopy Lie algebra, we transfer its structure on its homology and prove that the transferred structure is again a loop homotopy algebra. Moreover, we show that the homological perturbation lemma can be regarded as a path integral, integrating out the degrees of freedom which are not in the homology. The transferred action then can be interpreted as an effective action in the Batalin-Vilkovisky formalism. A review of necessary results from Batalin- Vilkovisky formalism and homotopy algebras is included as well. Powered by TCPDF (www.tcpdf.org)

Ce mémoire contient un résumé de mes travaux sur le thème des propérades et de leurs applications en algèbre, topologie, géométrie et physique mathématique.

Dans une première partie, on établit des résultats structuraux sur la construction cobar, visant à obtenir un relèvement homotopique explicite d'une structure de BV-algèbre sur la double construction cobar. Ces résultats interviennent à différentes itérations de la construction cobar. En conclusion, nous obtenons par descente de structures, un critère à l'obtention d'une structure de BV-algèbre homotopique (à la Gerstenhaber-Voronov) sur la double construction cobar Ω²C d'une G-cogèbre homotopique C, ceci en terme de co-opérations structurelles de C. Dans une seconde partie, nous appliquons le critère précédent sur la G-cogèbre homotopique C(X), où C(X) est le complexe de chaînes simpliciales sur un ensemble simplicial X. La structure de G-cogèbre homotopique considérée sur C(X) est telle que la double construction cobar Ω²C(X) est un modèle pour les lacets doubles Ω²|X|. Nous donnons ensuite des résultats de comparaisons entre la structure d'algèbre de Batalin-Vilkovisky obtenue sur la double construction cobar Ω²C(X) lorsque X est une double suspension et celle sur l'homologie H(Ω²|X|) induite par l'action diagonale du cercle sur Ω²|X|. Pour finir, lorsque l'anneau des coefficients est Q, nous déformons la structure de dg-algèbre de Hopf sur la construction cobar de Baues ΩC(X) en une structure de dg-algèbre de Hopf involutive (∇, S). On obtient alors une structure de BV-algèbre homotopique sur la double construction cobar Ω(ΩC(X), ∇, S) pour tout ensemble simplicial X.