This thesis consists of four papers on the topics of free loop spaces, Koszul duality and A∞-algebras.  In Paper I we consider a definition of differential operators for noncommutative algebras. This definition is inspired by the connections between differential operators of commutative algebras, L∞-algebras and BV-algebras. We show that the definition is reasonable by establishing results that are analoguous to results in the commutative case. As a by-product of this definition we also obtain definitions for noncommutative versions of Gerstenhaber and BV-algebras.  In Paper II we calculate the free loop space homology of (n-1)-connected manifolds of dimension of at least 3n-2. The Chas-Sullivan loop product and the loop bracket are calculated. Over a field of characteristic zero the BV-operator is determined as well. Explicit expressions for the Betti numbers are also established, showing that they grow exponentially.  In Paper III we restrict our coefficients to a field of characteristic 2. We study the Dyer-Lashof operations that exist on free loop space homology in this case. Explicit calculations are carried out for manifolds that are connected sums of products of spheres.  In Paper IV we extend the Koszul duality methods used in Paper II by incorporating A∞-algebras and A∞-coalgebras. This extension of Koszul duality enables us to compute free loop space homology of manifolds that are not necessarily formal and coformal. As an example we carry out the computations for a non-formal simply connected 7-manifold. / Denna avhandling består av fyra artiklar inom ämnena fria öglerum, Koszuldualitet och A∞-algebror. I Artikel I behandlar vi en definition av differentialoperatorer för ickekommutativa algebror. Denna definition är inspirerad av kopplingar mellan differentialoperatorer för kommutativa algebror, L∞-algebror och BV-algebror. Vi visar att definitionen är rimlig genom att etablera resultat som är analoga med resultat i det kommutativa fallet. Som en biprodukt får vi också definitioner för ickekommutativa varianter av Gerstenhaber och BV-algebror. I Artikel II beräknar vi den fria öglerumshomologin av (n-1)-sammanhängande mångfalder av dimension minst 3n-2. Chas-Sullivans ögleprodukt och öglehake beräknas. Över en kropp av karakteristik noll beräknas även BV-operatorn. Explicita uttryck för Bettitalen fastställs också, vilka visar att de växer exponentiellt. I Artikel III begränsar vi koefficienterna till en kropp av karakteristik 2. Vi studerar Dyer- Lashofoperationer som existerar på den fria öglerumshomologin i detta fall. Explicita beräkningar görs för mångfalder som är sammanhängande summor av produkter av sfärer. I Artikel IV utvidgar vi Koszuldualitetmetoden som används i Artikel II genom att inkorporera A∞-algebror och A∞-koalgebror. Denna utvidgning av Koszuldualitet gör det möjligt att beräkna fri öglerumshomologi för mångfalder som inte nödvändigtvis är formella och koformella. Som ett exempel utför vi beräkningar för en ickeformell enkelt sammanhängande 7-mångfald. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript.</p>

Le mappe comomento omotopiche sono una generalizzazione della nozione di mappa momento introdotta al fine di estendere il concetto di azione hamiltoniana al contesto della geometria multisimplettica. L'obiettivo di questa tesi è fornire nuove costruzioni esplicite ed esempi concreti di azioni di gruppi di Lie su varietà multisimplettiche che ammettono delle mappe comomento. Il primo risultato è una classificazione completa delle azioni di gruppi compatti su sfere multisimplettiche. In questo caso, l'esistenza di mappe comomento omotopiche dipende dalla dimensione della sfera e dalla transitività dell'azione di gruppo. Il secondo risultato è la costruzione esplicita di un analogo multisimplettico dell’inclusione dell'algebra di Poisson di una varietà simplettica dentro il corrispondente algebroide di Lie twistato. E’ possibile dimostrare che questa inclusione soddisfa una relazione di compatibilità nel caso di varietà multisimplettiche gauge-correlate in presenza di un'azione di gruppo Hamiltoniana. Tale costruzione potrebbe giocare un ruolo nella formulazione di un analogo multisimplettico della procedura di quantizzazione geometrica. L’ultimo risultato è una costruzione concreta di una mappa comomento omotopica relativa all'azione multisimplettica del gruppo di diffeomorfismi che preservano la forma volume dello spazio Euclideo. Questa mappa ammette naturalmente un’interpretazione idrodinamica, nello specifico trasgredisce alla mappa comomento idrodinamica introdotta da Arnol'd, Marsden, Weinstein e altri. La mappa comomento così costruita può essere inoltre messa in relazione alla teoria dei nodi avvalendosi dell’approccio ai link nel formalismo dei vortici. Questo punto di apre la strada a un'interpretazione semiclassica del polinomio HOMFLYPT nel linguaggio della quantizzazione geometrica. / Homotopy comomentum maps are a higher generalization of the notion of moment map introduced to extend the concept of Hamiltonian actions to the framework of multisymplectic geometry. Loosely speaking, higher means passing from considering symplectic $2$-form to consider differential forms in higher degrees. The goal of this thesis is to provide new explicit constructions and concrete examples related to group actions on multisymplectic manifolds admitting homotopy comomentum maps. The first result is a complete classification of compact group actions on multisymplectic spheres. The existence of a homotopy comomentum maps pertaining to the latter depends on the dimension of the sphere and the transitivity of the group action. Several concrete examples of such actions are also provided. The second novel result is the explicit construction of the higher analogue of the embedding of the Poisson algebra of a given symplectic manifold into the corresponding twisted Lie algebroid. It is also proved a compatibility condition for such embedding for gauge-related multisymplectic manifolds in presence of a compatible Hamiltonian group action. The latter construction could play a role in determining the multisymplectic analogue of the geometric quantization procedure. Finally a concrete construction of a homotopy comomentum map for the action of the group of volume-preserving diffeomorphisms on the multisymplectic 3-dimensional Euclidean space is proposed. This map can be naturally related to hydrodynamics. For instance, it transgresses to the standard hydrodynamical co-momentum map of Arnol'd, Marsden and Weinstein and others. A slight generalization of this construction to a special class of Riemannian manifolds is also provided. The explicitly constructed homotopy comomentum map can be also related to knot theory by virtue of the aforementioned hydrodynamical interpretation. Namely, it allows for a reinterpretation of (higher-order) linking numbers in terms of multisymplectic conserved quantities. As an aside, it also paves the road for a semiclassical interpretation of the HOMFLYPT polynomial in the language of geometric quantization.

La théorie de Hodge mixte de Deligne fournit des structures supplémentaires sur les groupes de cohomologie des variétés algébriques complexes. Depuis, des structures de Hodge mixtes ont été construites sur les groupes d'homotopie rationnels de telles variétés par Morgan et Hain. Dans cette lignée, nous construisons des structures de Hodge mixtes sur des invariants associés aux représentations linéaires des groupes fondamentaux des variétés algébriques complexes lisses. Le point de départ est la théorie de Goldman et Millson qui relie la théorie des déformations de telles représentations à la théorie des déformations via les algèbres de Lie différentielles graduées. Ceci a été relu par P. Eyssidieux et C. Simpson dans le cas des variétés kählériennes compactes. Dans le cas non compact, et pour des représentations d'image finie, Kapovich et Millson ont construit seulement des graduations non canoniques. Pour construire des structures de Hodge mixtes dans le cas non compact et l'unifier avec le cas compact traité par Eyssidieux-Simpson, nous ré-écrivons la théorie de Goldman-Millson classique en utilisant des idées plus modernes de la théorie des déformations dérivée et une construction d'algèbres L-infini due à Fiorenza et Manetti. Notre structure de Hodge mixte provient alors directement du H^0 d'un complexe de Hodge mixte explicite, de façon similaire à la méthode de Hain pour le groupe fondamental, et dont la fonctorialité apparaît clairement. / The mixed Hodge theory of Deligne provides additional structures on the cohomology groups of complex algebraic varieties. Since then, mixed Hodge structures have been constructed on the rational homotopy groups of such varieties by Morgan and Hain. In this vein, we construct mixed Hodge structures on invariants associated to linear representations of fundamental groups of smooth complex algebraic varieties. The starting point is the theory of Goldman and Millson that relates the deformation theory of such representations to the deformation theory via differential graded Lie algebras. This was reviewed by P. Eyssidieux and C. Simpson in the case of compact Kähler manifolds. In the non-compact case, and for representations with finite image, Kapovich and Millson constructed only non-canonical gradings. In order to construct mixed Hodge structures in the non-compact case and unify it with the compact case treated by Eyssidieux-Simpson, we re-write the classical Goldman-Millson theory using more modern ideas from derived deformation theory and a construction of L-infinity algebras due to Fiorenza and Manetti. Our mixed Hodge structure comes then directly from the H^0 of an explicit mixed Hodge complex, in a similar way as the method of Hain for the fundamental group, and whose functoriality appears clearly.