Higher genus counterexamples to relative Manin{Mumford Sean Howe
[email protected] Advised by prof. dr. S.J. Edixhoven. ALGANT Master's Thesis { Submitted 10 July 2012 Universiteit Leiden and Universite´ Paris{Sud 11 Contents Chapter 1. Introduction 2 Chapter 2. Preliminaries 3 2.1. Conventions and notation 3 2.2. Algebraic curves 3 2.3. Group schemes 4 2.4. The relative Picard functor 5 2.5. Jacobian varieties 8 Chapter 3. Pinching, line bundles, and Gm{extensions 11 3.1. Amalgamated sums and pinching 11 3.2. Pinching and locally free sheaves 17 3.3. Pinching and Gm{extensions of Jacobians over fields 26 Chapter 4. Higher genus counterexamples to relative Manin{Mumford 28 4.1. Relative Manin{Mumford 28 4.2. Constructing the lift 30 4.3. Controlling the order of lifts 32 4.4. Explicit examples 33 Bibliography 35 1 CHAPTER 1 Introduction The goal of this work is to provide a higher genus analog of Edixhoven's construction [2, Appendix] of Bertrand's [2] counter-example to Pink's relative Manin{Mumford conjecture (cf. Section 4.1 for the statement of the conjecture). This is done in Chapter 4, where further discussion of the conjecture and past work can be found. To give our construction, we must first develop the theory of pinchings a la Ferrand [8] in flat families and understand the behavior of the relative Picard functor for such pinchings, and this is the contents of Chapter 3. In Chapter 2 we recall some of the results and definitions we will need from algebraic geometry; the reader already familiar with these results should have no problem beginning with Chapter 3 and referring back only for references and to fix definitions.