Béranger Seguin

Let K be a field of characteristic 0 and k≥2 be an integer. We prove that every K-linear bijection f:K[X]→K[X] strongly preserving the set of k-free polynomials (or the set of polynomials with a k-fold root in K) is a constant multiple of a K-algebra automorphism of K[X], i.e., there are elements a,c∈Kˣ, b∈K such that f(P)(X) = cP(aX+b). When K is a number field or K=ℝ, we prove that similar statements hold when f preserves the set of polynomials with a root in K.

For a finite group G, we describe the asymptotical growth of the number of connected components of Hurwitz spaces of marked G-covers (of both the affine and projective lines) whose monodromy classes are constrained in a certain way, as the number of branch points grows to infinity. More precisely, we compute both the exponent and (in many cases) the coefficient of the leading monomial in the count of components containing covers whose monodromy group is a given subgroup of G. By the work of Ellenberg, Tran, Venkatesh and Westerland, this asymptotical behavior is related to the distribution of field extensions of 𝔽q(T) with Galois group G.

We answer various questions concerning the distribution of extensions of a given central simple algebra K over a number field. Specifically, we give asymptotics for the count of inner Galois extensions L|K of fixed degree and center with bounded discriminant. We also relate the distribution of outer extensions of K to the distribution of field extensions of its center Z(K). This paper generalizes the study of asymptotics of field extensions to the noncommutative case in an analogous manner to the program initiated by Deschamps and Legrand to extend inverse Galois theory to skew fields.

We study a weak divisibility property for noncommutative rings: a nontrivial ring is fadelian if for all nonzero a and x there exist b, c such that x=ab+ca. We prove properties of fadelian rings and construct examples thereof which are not division rings, as well as non-Noetherian and non-Ore examples.

Hurwitz spaces are moduli spaces that classify ramified covers of the projective line on which a fixed group G acts. Their geometric and arithmetic properties are related to number theoretical questions, particularly the inverse Galois problem. In this thesis, we study the connected components of these spaces. Firstly, we prove results concerning the asymptotical behaviour of the count of connected components of Hurwitz spaces as the number of branch points of the covers they classify grows. Secondly, we establish stability results for fields of definitions of connected components of Hurwitz spaces under the gluing operation. These results relate topological and arithmetical properties of covers. Three expository chapters, devoid of original statements, present the various objects. In an appendix, we summarize the thesis for the general public.

For a fixed finite group G, we study the fields of definition of geometrically irreducible components of Hurwitz moduli schemes of marked branched G-covers of the projective line. The main focus is on determining whether components obtained by "gluing" two other components, both defined over a number field K, are also defined over K. The article presents a list of situations in which a positive answer is obtained. Applications are given.

We study a variant of the ring of components of Hurwitz moduli spaces for covers, introduced by Ellenberg, Venkatesh and Westerland. For G-covers of the projective line, the ring of components is a commutative algebra of finite type. We study it using tools from algebraic geometry. We relate the geometry of its spectrum to group-theoretical properties and combinatorial aspects of Galois covers. When G is a symmetric group, we fully describe the geometric points of the spectrum.