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In this paper we use the theory of formal moduli problems developed by Lurie in order to study the space of formal deformations of a $k$-linear $\infty$-category for a field $k$. Our main result states that if $\mathcal{C}$ is a $k$-linear $\infty$-category which has a compact generator whose groups of self extensions vanish for sufficiently high positive degrees, then every formal deformation of $\mathcal{C}$ has zero curvature and moreover admits a compact generator., Comment: Preliminary version. Comments are welcome. 40p

In this thesis, we give a definition of topological K-theory of Kontsevich's noncommutative spaces (ie dg-categories) defined over the complex. The main motivation comes from noncommutative Hodge structures in the sense of Kontsevich--Katzarkov--Pantev on the periodic cyclic homology of smooth and proper dg-algebras. The essential ingredient in the definition is the topological realization functor from presheaves of spectra on the site of complex affine schemes to spectra. Topological K-theory is defined as the Bott inverted topological realization of nonconnective algebraic K-theory. Using a non-abelian generalization of Deligne's proper cohomological descent, we show that the topological realization of the preaheave given by nonconnective K-theory is the spectrum bu of connective topological K-theory. The other main result deals with the relation between topological K-theory of a dg-category T and the moduli stack of perfect modules over T^op. Moreover, topological K-theory can be endowed with a Chern character map to periodic cyclic homology which factorizes Cisinski--Tabuada Chern map. We give a comparison result for smooth schemes of finite type over the complex and for finite dimensional associative algebras., Comment: In French

The purpose of this work is to give a definition of a topological K-theory for dg-categories over C and to prove that the Chern character map from algebraic K-theory to periodic cyclic homology descends naturally to this new invariant. This topological Chern map provides a natural candidate for the existence of a rational structure on the periodic cylic homology of a smooth proper dg-algebra, within the theory of noncommutative Hodge structures. The definition of topological K-theory consists in two steps : taking the topological realization of algebraic K-theory, and inverting the Bott element. The topological realization is the left Kan extension of the functor "space of complex points" to all simplicial presheaves over complex algebraic varieties. Our first main result states that the topological K-theory of the unit dg-category is the spectrum BU. For this we are led to prove a homotopical generalization of Deligne's cohomological proper descent, using Lurie's proper descent. The fact that the Chern character descends to topological K-theory is established by using Kassel's K\"unneth formula for periodic cyclic homology and once again the proper descent result. In the case of a dg-category of perfect complexes on a smooth scheme, we show that we recover the usual topological K-theory. Finally in the case of a finite dimensional associative algebra, we show that the lattice conjecture holds. This gives a formula for the periodic homology groups of a finite dimensional algebra in terms of the stack of projective modules of finite type., Comment: 72 pages. To appear in Compositio Mathematica. Corrected the assumptions in the comparison isomorphism for schemes (now stated for singular schemes, proposition 4.32) as well as some technical assumptions in proposition 3.24