Combinatorics and degenerations in algebraic geometry: Hurwitz numbers, Mustafin varieties and tropical geometry

Degenerationstechniken sind mächtige Werkzeuge in der Untersuchung verschiedener Objekte der algebraischen Geometrie. Sie ermöglichen es geometrische Objekte mit Hilfe kombinatorischer Methoden zu untersuchen. Diesem großen Nutzen geht jedoch häufig die große Herausforderung voraus den Zusammenhang zur Kombinatorik zu konkretisieren. Das junge Gebiet der tropischen Geometrie bietet einen konzeptionellen Rahmen, um diesen Zusammenhang herzustellen. In dieser Arbeit untersuchen wir verschiedene Probleme der algebraischen Geometrie mit Hilfe degenerativer und kombinatorischer Methoden. Diese Probleme lassen sich wiederum in drei Themengebiete unterteilen: Enumeration von Überlagerungen (Hurwitz Zahlen), Degenerationen von projektiven Räumen (Mustafin varietäten), Modulräume tropischer Kurven und treue Tropikalisierungen

Triply mixed coverings of arbitrary base curves: Quasimodularity, quantum curves and a mysterious topological recursions

Simple Hurwitz numbers enumerate branched morphisms between Riemann surfaces with fixed ramification data. In recent years, several variants of this notion for genus $0$ base curves have appeared in the literature. Among them are so-called monotone Hurwitz numbers, which are related to the HCIZ integral in random matrix theory and strictly monotone Hurwitz numbers which count certain Grothendieck dessins d'enfants. We generalise the notion of Hurwitz numbers to interpolations between simple, monotone and strictly monotone Hurwitz numbers to any genus and any number of arbitrary but fixed ramification profiles. This yields generalisations of several results known for Hurwitz numbers. When the target surface is of genus one, we show that the generating series of these interpolated Hurwitz numbers are quasimodular forms. In the case that all ramification is simple, we refine this result by writing this series as a sum of quasimodular forms corresonding to tropical covers weighted by Gromov-Witten invariants. Moreover, we derive a quantum curve for monotone and Grothendieck dessins d'enfants Hurwitz numbers for arbitrary genera and one arbitrary but fixed ramification profile. Thus, we obtain spectral curves via the semiclassical limit as input data for the CEO topological recursion. Astonishingly, we find that the CEO topological recursion for the genus $1$ spectral curve of the strictly monotone Hurwitz numbers compute the monotone Hurwitz numbers in genus $0$. Thus, we give a new proof that monotone Hurwitz numbers satisfy CEO topological recursion. This points to an unknown relation between those enumerants. Finally, specializing to target surface $\mathbb{P}^1$, we find recursions for monotone and Grothendieck dessins d'enfants double Hurwitz numbers, which enables the computation of the respective Hurwitz numbers for any genera with one arbitrary but fixed ramification profile.Comment: 41 page