We consider a variant of the ring of components of Hurwitz spaces introduced by Ellenberg, Venkatesh and Westerland.
By focusing on Hurwitz spaces classifying covers of the projective line, the resulting ring of components is commutative, which lets us study it from the point of view of algebraic geometry and relate its geometric properties to numerical invariants involved in our previously obtained asymptotic counts.
Specifically, we describe a stratification of the prime spectrum of the ring of components, and we compute the dimensions and degrees of the strata.
Using the stratification, we give a complete description of the spectrum in some cases.
@preprint{geomringcomp,author={Seguin, Béranger},title={The Geometry of Rings of Components of Hurwitz Spaces},year={2024},month=oct,eprint={arXiv:2210.12793},note={25 pages.}}
article
Counting Components of Hurwitz Spaces
Béranger
Seguin
Sep 2024
Accepted for publication in the Israel Journal of Mathematics. 22 pages.
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.
@article{countcomp,author={Seguin, Béranger},title={Counting Components of Hurwitz Spaces},year={2024},month=sep,note={Accepted for publication in the <i>Israel Journal of Mathematics</i>. 22 pages.},eprint={arXiv:2409.18246},}
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.
@preprint{sympol,author={Seguin, Béranger},title={Symmetries of various sets of polynomials},year={2024},month=jul,eprint={arXiv:2407.09118},note={17 pages.}}
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.
@preprint{asympskew,author={Gundlach, Fabian and Seguin, Béranger},title={Asymptotics of extensions of simple ℚ-algebras},year={2024},month=may,eprint={arXiv:2405.17286},note={32 pages.}}
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.
@article{fadrings,author={Khanfir, Robin and Seguin, Béranger},title={Study of a division-like property},year={2024},month=apr,doi={10.1142/S0219498825502214},journal={Journal of Algebra and its Applications},}
2023
report
Covers of ℙ¹ and their moduli: where arithmetic, geometry and combinatorics meet
@inproceedings{mforeport,title={Covers of ℙ¹ and their moduli: where arithmetic, geometry and combinatorics meet},author={Seguin, Béranger},year={2023},month=sep,booktitle={<a href="https://publications.mfo.de/bitstream/handle/mfo/4128/OWR_2023_42.pdf">MFO-RIMS Tandem Workshop: Arithmetic Homotopy and Galois Theory</a> (pages 2400-2404)},doi={10.4171/OWR/2023/42},address={Mathematisches Forschungsinstitut Oberwolfach},}
PhD thesis
Geometry and Arithmetic of Components of Hurwitz Spaces
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.
@phdthesis{these,author={Seguin, Béranger},title={Geometry and Arithmetic of Components of Hurwitz Spaces},year={2023},month=jul,}
preprint
Fields of Definition of Components of Hurwitz Spaces
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.
@preprint{fielddef,author={Seguin, Béranger},title={Fields of Definition of Components of Hurwitz Spaces},year={2023},month=mar,eprint={arXiv:2303.05903},note={28 pages.}}