Rigid analytic curves and their Jacobians Dissertation zur Erlangung des Doktorgrades Dr. rer. nat. der Fakult¨at f¨urMathematik und Wirtschaftswissenschaften der Universit¨atUlm vorgelegt von Sophie Schmieg aus Ebersberg Ulm 2013 Erstgutachter: Prof. Dr. Werner Lutkebohmert¨ Zweitgutachter: Prof. Dr. Stefan Wewers Amtierender Dekan: Prof. Dr. Dieter Rautenbach Tag der Promotion: 19. Juni 2013 Contents Glossary of Notations vii Introduction ix 1. The Jacobian of a curve in the complex case . ix 2. Mumford curves and general rigid analytic curves . ix 3. Outline of the chapters and the results of this work . x 4. Acknowledgements . xi 1. Some background on rigid geometry 1 1.1. Non-Archimedean analysis . 1 1.2. Affinoid varieties . 2 1.3. Admissible coverings and rigid analytic varieties . 3 1.4. The reduction of a rigid analytic variety . 4 1.5. Adic topology and complete rings . 5 1.6. Formal schemes . 9 1.7. Analytification of an algebraic variety . 11 1.8. Proper morphisms . 12 1.9. Etale´ morphisms . 13 1.10. Meromorphic functions . 14 1.11. Examples . 15 2. The structure of a formal analytic curve 17 2.1. Basic definitions . 17 2.2. The formal fiber of a point . 17 2.3. The formal fiber of regular points and double points . 22 2.4. The formal fiber of a general singular point . 23 2.5. Formal blow-ups . 27 2.6. The stable reduction theorem . 29 2.7. Examples . 31 3. Group objects and Jacobians 33 3.1. Some definitions from category theory . 33 3.2. Group objects . 35 3.3. Central extensions of group objects . 37 3.4. Algebraic and formal analytic groups . 41 3.5. Extensions by tori . 44 4. The Jacobian of a formal analytic curve 51 4.1. The cohomology of graphs . 51 4.2. The cohomology of curves with semi-stable reduction . 54 v 4.3. The Jacobian of a semi-stable curve . 56 4.4. The Jacobian of a curve with semi-stable reduction . 60 4.5. Examples . 66 A. Bibliography 71 vi Glossary of Notations We denote by + R the positive real numbers as a multiplicative group + R0 the non-negative real numbers Q(R) the quotient field of a ring R R˚ the subring fx 2 R ; jxj ≤ 1g of a normed ring R Rˇ the ideal fx 2 R ; jxj < 1g of R˚ where R is a normed ring K a valued field R the ring K˚ k the field K=˚ Kˇ Gm;K the multiplicative group of K, seen as an analytic variety ¯ Gm;K the group fx 2 K ; jxj = 1g seen as a formal analytic variety Gm;k the multiplicative group of k, seen as an algebraic variety n BK the affinoid analytic variety Sp Khζ1; : : : ; ζni X;~ f;~ x~ the reductions of the corresponding formal analytic counterpart. vii Introduction In this work, we describe the general structure of a rigid analytic curve and its Jacobian. 1. The Jacobian of a curve in the complex case The study of the Jacobian of an algebraic curve started with the research of certain integrals that appear in the calculation of the circumference of an ellipse. Niels Henrik Abel and Carl Gustav Jacob Jacobi first described the Jacobian variety around 1826. Of course, they could not formulate it in terms of algebraic curves, since it was Bernhard Riemann, a good 25 years later, who first defined the Riemann surface and thereby described algebraic curves over C. It took many more men and women to arrive at the modern description of this theory in the middle of the twentieth century. The Jacobian variety of an algebraic curve of genus g is the set of line bundles of degree zero over this curve. The tensor product gives this set the natural structure of a group. A lot harder to show, but in the same way natural is its structure as an algebraic variety of dimension g itself, containing the original curve as a closed subvariety. The Jacobian variety is therefore an Abelian variety with a canonical polarization, deriving from the embedding of the curve. Over C the Jacobian variety of a curve of genus g can be described as H0(X; Ω1 )0=H (X; ), a quotient of g by a lattice M of rank 2g, which X=C 1 Z C g g is generated by certain integrals on the curve. We can write M = Z ⊕ ZZ , so applying the exponential function let us write g = exp(2πiZ). So the Ja- Gm;C cobian of a Riemann surface is the multiplicative group of C to the power g modulo a lattice of rank g. 2. Mumford curves and general rigid analytic curves The complex numbers are just one possibility to create a topological and alge- −ν(x) braic closure of Q. For every prime p we can define jxj = p , where ν is the valuation associated to p. This was first described by Kurt Hensel and later refined by his student Helmut Hasse at the end of the eighteenth century. The topological closure of Q with this absolute value is the field Qp of p-adic num- bers. Its algebraic closure Cp has infinite degree over Qp. There is no equivalent of the exponential function on Cp and the topology has very strange properties. Furthermore, the p-adic value gives rise to the reduction functor giving a close relation to the finite field Fp and its algebraic closure. The description of the Jacobian variety of a rigid analytic curve, i.e. a curve over Qp mainly decomposes into two parts, a combinatorial one and a so-called ix formal one. Omitting the formal part, one can describe the Jacobian of Mum- g ford curves, where the reduction has a certain simple form, as Gm;K =M, where M is a lattice of rank g, and thus showing a wonderful analogy to the com- plex case. Since the integral can not be defined over the p-adic numbers in a meaningful way, one needs to replace the classic formulation of the Riemann relations with a more general, cohomological one. To describe the Jacobian of a general rigid analytic curve, one needs to work with Raynaud extensions, heavily researched by Michel Raynaud, Siegfried Bosch and Werner Lutkebohmert¨ . Then one realizes that the Jacobian of a rigid analytic curve can be written as E=M, where M is a lattice of rank g and E is an extension t 0 ! Gm;K ! E ! B ! 0 t of a formal analytic abelian variety B by the torus Gm;K . 3. Outline of the chapters and the results of this work In the first chapter, we will recapitulate the basic facts about rigid analytic varieties, their topology and their relationship with formal analytic schemes of locally topologically finite type. This chapter is by no means a complete introduction into the topic, we refer to [BGR84] and [Bos05] for this. In the second chapter, we will provide some new insight into the stable reduc- tion theorem, by refining the proofs of a few theorems of [BL85], using much shorter and less technical arguments than the original work. First, we will show that the ring O˚X (X+(~x)) of bounded functions on the formal fiber of a pointx ~ of the reduction is local and henselian, by taking a close look at the normalization ofx ~. Secondly we will then be able to give a much more natural proof for the Theorem 2.4.1 which describes the periphery of a formal fiber. That periph- ery always consists of a disjoint union of annuli, which we can equate to the structure of the normalization of the curve at the pointx ~. Finally, we will describe how this theorem is used to get to the stable reduc- tion theorem. In the third chapter, we will introduce group objects over an arbitrary cate- gory. This allows us to form a general theory describing algebraic, rigid analytic and formal analytic groups simultaneously. With this theory, we can generalize the results of [Ser88] and describe how an extension t 0 ! Gm;K ! E ! B ! 0 t of an analytic or algebraic group B by the torus Gm;K equals to a line bundle over B. The work of the third chapter pays off in the fourth and final chapter, where we can give the explicit description of the lattice M which E gets divided by to form the Jacobian variety of our curve. While it was known that such a lattice exists and has full rank, we can even give a constructive formula for it. It will be shown that the formal analytic variety B does not influence the absolute x value of the lattice and that one gains the explicit formula for the generators t vi = (vij)j=1 of the lattice as X vij = −dn · logjqnj en2γi\γj where qn is the height of the formal fiber of the double point corresponding to en and dn = 1 if en has the same orientation in γi as in γj and dn = −1 otherwise and where γi and γj are simple cycles on the curve, in close relationship to the complex case. The lattice can be described fully by this method, but the description depends on the structure of the variety B of which little is known. We can construct B explicitly for the special family of curves X, which have a reduction consisting of a rational curve and curve Y~ of genus g, together with a surjective map ': X ! Y , with Y being a lift of Y~ . Then it turns out that B is isogenic to Jac Y of the same degree as the map '.