# PhD students of Steve Vickers: Ming Ng

Ming Ng completed his PhD thesis "Adelic Geometry via Topos Theory" in 2022 at the School of Computer Science, University of Birmingham, under my supervision.

Some of the results have been written up as papers, "Point-free construction of real exponentiation".

### THESIS ABSTRACT

#### "Adelic Geometry via Topos Theory"

PhD Thesis, School of Computer Science, University of Birmingham, 2022. 324 pages.

Summary

Our starting point has to do with a key tension running through number theory: although all completions of the rationals Q should be treated symmetrically, this is complicated by fundamental disanalogies between the p-adics vs. the reals. Whereas prior work has typically been guided by classical point-set reasoning, this thesis explores various ways of pulling this problem away from the underlying set theory, revealing various surprises that are obscured by the classical perspective. Framing these investigations is the following test problem: construct and describe the topos of completions of Q (up to equivalence).

Chapter 2 begins with the preliminaries: we set up the topos-theoretic framework of point-free topology, with a view towards highlighting the distinction between classical vs. geometric mathematics, before introducing the number-theoretic context. A key theme is that geometric mathematics possesses an intrinsic continuity, which forces us to think more carefully about the topological character of classical algebraic constructions.

Chapter 3 represents the first step towards constructing the topos of completions. Here, we provide a pointfree account of real exponentiation and logarithms, which will allow us to define the equivalence of completions geometrically. Chapter 4 provides a geometric proof of Ostrowski's Theorem for both upper-valued abosolute values on Z as well as Dedekind-valued absolute values on Q, along with some key insights about the relationship between the multiplicative seminorms and upper reals.

In a slightly more classical interlude, Chapter 5 extends these insights to obtain a surprising generalisation of a foundational result in Berkovich geometry. Namely, by replacing the use of classical rigid discs with formal balls, we obtain a classification of the points of Berkovich Spectra M(K{R^{-1}T}) via the language of filters [more precisely, what we call: R-good filters] even when the base field K is trivially-valued.

Returning to geometricity, Chapter 6 builds upon Chapters 3 and 4 to investigate the space of places of Q via descent arguments. Here, we uncover an even deeper surprise. Although the non-Archimedean places correspond to singletons (as is classically expected), the Archimedean place corresponds to the subspace of upper reals in [0, 1], a sort of blurred unit interval. The chapter then analyses the topological differences between the non-Archimedean vs. Archimedean places. In particular, we discover that while the topos corresponding to Archimedean place witnesses non-trivial forking in the connected components of its sheaves, the topos corresponding to the non-Archimedean place eliminates all kinds of forking phenomena. We then conclude with some insights and observations, framed by the question: "How should the connected and the disconnected interact?"