Diffeological and differential spaces are generalisations of smooth structures on manifolds. We show that the “intersection” of these two categories is isomorphic to Frölicher spaces, another generalisation of smooth structures. We then give examples of such spaces, as well as examples of diffeological and differential spaces that do not fall into this category. We apply the theory of diffeological spaces to differential forms on a geometric quotient of a compact Lie group. We show that the subcomplex of basic forms is isomorphic to the complex of diffeological forms on the geometric quotient. We apply this to symplectic quotients coming from a regular value of the momentum map, and show that diffeological forms on this quotient are isomorphic as a complex to Sjamaar differential forms. We also compare diffeological forms to those on orbifolds, and show that they are isomorphic complexes as well. We apply the theory of differential spaces to subcartesian spaces equipped with families of vector fields. We use this theory to show that smooth stratified spaces form a full subcategory of subcartesian spaces equipped with families of vector fields. We give families of vector fields that induce the orbit-type stratifications induced by a Lie group action, as well as the orbit-type stratifications induced by a Hamiltonian group action.

Diffeological and differential spaces are generalisations of smooth structures on manifolds. We show that the “intersection” of these two categories is isomorphic to Frölicher spaces, another generalisation of smooth structures. We then give examples of such spaces, as well as examples of diffeological and differential spaces that do not fall into this category. We apply the theory of diffeological spaces to differential forms on a geometric quotient of a compact Lie group. We show that the subcomplex of basic forms is isomorphic to the complex of diffeological forms on the geometric quotient. We apply this to symplectic quotients coming from a regular value of the momentum map, and show that diffeological forms on this quotient are isomorphic as a complex to Sjamaar differential forms. We also compare diffeological forms to those on orbifolds, and show that they are isomorphic complexes as well. We apply the theory of differential spaces to subcartesian spaces equipped with families of vector fields. We use this theory to show that smooth stratified spaces form a full subcategory of subcartesian spaces equipped with families of vector fields. We give families of vector fields that induce the orbit-type stratifications induced by a Lie group action, as well as the orbit-type stratifications induced by a Hamiltonian group action.

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.