O-minimality, nonclassical modular functions and diophantine problems

Thesis

There now exists an abundant collection of conjectures and results, of various complexities, regarding the diophantine properties of Shimura varieties. Two central such statements are the Andre-Oort and Zilber-Pink Conjectures, the first of which is known in many cases, while the second is known in very few cases indeed.

The motivating result for much of this document is the modular case of the Andre-Oort Conjecture, which is a theorem of Pila. It is most commonly viewed as a statement about the simplest kind of Shimura varieties, namely modular curves. Here, we tend instead to view it as a statement about the properties of the classical modular j-function. It states, given a complex algebraic variety V, that V contains only finitely many maximal special subvarieties, where a special variety is one which arises from the arithmetic behaviour of the j-function in a certain natural way.

The central question of this thesis is the following: what happens if in such statements we replace the j-function with some other kind of modular function; one which is less well-behaved in one way or another? Such modular functions are naturally called nonclassical modular functions. This question, as we shall see, can be studied using techniques of o-minimality and point-counting, but some interesting new features arise and must be dealt with.

After laying out some of the classical theory, we go on to describe two particular types of nonclassical modular function: almost holomorphic modular functions and quasimodular functions (which arise naturally from the derivatives of the j-function). We go on to prove some results about the diophantine properties of these functions, including several natural Andre-Oort-type theorems, then conclude by discussing some bigger-picture questions (such as the potential for nonclassical variants of, say, Zilber-Pink) and some directions for future research in this area.

Authors: Spence, H

• DOI: 10.5287/ora-vjmkbwjer
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Modular Functions; Number Theory; O-minimality; Logic; Pila-Wilkie Theorem; Shimura Varieties; Unlikely Intersections; Model Theory; Andre-Oort; Diophantine Problems; Zilber-Pink
• Subjects: Number Theory; Logic; Pure Mathematics