DOCSLIB.ORG

CURVES NOTES Eric Larson Taught a Course On

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
CURVES NOTES Eric Larson Taught a Course On CURVES NOTES AARON LANDESMAN 1. INTRODUCTION Eric Larson taught a course on Curves to projective space at Stanford in Fall 2019. These are my “live-TeXed“ notes from the course. Conventions are as follows: Each lecture gets its own “chapter,” and appears in the table of contents with the date. Of course, these notes are not a faithful representation of the course, either in the mathematics itself or in the quotes, jokes, and philosophical musings; in particular, the errors are my fault. By the same token, any virtues in the notes are to be credited to the lecturer and not the scribe.1 Please email suggestions to [email protected] 2. CONVENTIONS Here are some conventions we will adapt throughout the notes. (1) I have a preference toward making any detail stated in class, which is not verified, into an exercise. This will mean there are many trivial exercises in the notes. When the exercise seems nontrivial, I will try to give a hint. Feel free to contact me if there are any exercises you do not know how to solve or other details which are unclear. (2) Throughout the notes, I will often include parenthetical remarks which describe things beyond the scope of the course. If some word or explanation is placed in quotation marks or in parentheses, and you don’t understand it, don’t worry about it! It’s more meant to give you a flavor of how one might describe the 19th century ideas in this course in terms of 20th century algebraic geometry. newpage 1This introduction has been adapted from Akhil Matthew’s introduction to his notes, with his permission. 1 2 AARON LANDESMAN 3. 9/23/19 3.1. Goals of course. Goal 3.1. The goal throughout this term is to understand the geometry of curves in projective space. There are three overarching questions: (1) What are the degrees and genera of curves in projective space. I.e., what possible types of curves are there? (2) What is the geometry of the moduli space of such curves? (3) What is the geometry of an individual such curve? 3.2. Logistics. Office hours will occur Monday and Tuesdays between 1:30 and 3. Grading for the class for undergrads will be based on solutions to homework problems which will be posed in lecture. Graduate students are encouraged to think about these homework problems, but do not need to turn anything in. To the extent that there is a textbook for this course (though we won’t be following it much) about half of what we’ll discuss can be found in Moduli of Curves by Harris and Morrison. 3.3. Maps from curves to projective space. Remark 3.2. For the entire duration of this class, we will always work over the complex numbers. Question 3.3. How does one describe a curve in projective space? Recall that for C an algebraic curve, a map C ! Pr is the same as the 0 datum of a line bundle L on C together with x1, ... , xr+1 2 H (C, L ) which are basepoint free, i.e., there is no p 2 C with xi(p) = 0 for all i. Question 3.4. How do we calculate H0(C, L )? This naturally connects to the topic of the dualizing sheaf. 3.4. The Dualizing sheaf. Let C be a smooth irreducible curve of genus g. Later, we will relax this assumption. Let p1, ... , pd 2 C be distinct points. This is for ease of notation, and they can often coincide. 0 Question 3.5. What is dim H (C, OC (p1 + ··· + pd))? A section of OC(p1 + ··· + pd) is determined by its principal parts at p1,..., pd. Up to the addition of a constant. Question 3.6. Which principal parts can be completed to a section? CURVES NOTES 3 0 0 If we have a 2 H (KC) and s 2 H (OC(p1 + ··· + pd)) we must have ( · ) = ∑ Respi a s 0. i This condition is trivial if and only if a vanishes at p1, ... , pd. This yields the following lemma. Lemma 3.7. 0 0 0 dim H (OC (p1 + ··· + pd)) ≤ d + 1 − dim H (KC) − dim H (KC (−p1 − · · · − pd)) . Equivalently, 0 0 dim H (OC (p1 + ··· + pd)) − dim H (KC (−p1 − · · · − pd)) ≤ d + 1 − g. Theorem 3.8 (Riemann Roch and Serre Duality). The inequality in Lemma 3.7 is an equality. That is 0 0 dim H (OC (p1 + ··· + pd)) − dim H (KC (−p1 − · · · − pd)) = d + 1 − g. Remark 3.9. Riemann Roch is saying that the obstructions to constructing sections is precisely the condition that the residues when paired with any differential form sum to 0. Remark 3.10. Alternatively, we have a duality map 0 1 H (KC (−p1 − · · · − pd)) × H (OC (p1 + ··· + pd)) ! C.(3.1) We can then choose small disjoint neighborhoods Di of pi which pairwise do not intersect. Let U = C − fp1, ... , pdg. We can think of an element of 1 0 H (OC (p1 + ··· + pd)) by a 1-cocycle fi 2 H ODi−pi . Then, the pairing above from Serre duality (3.1) is given by ( f g) 7! ( · ) g, fi ∑ Respi g fi . i Remark 3.11. We’ll primarily be interested in smooth curves, but they fit into a moduli space containing singular curves, so it will be useful to carry out many ideas for singular curves. Remark 3.12. The same logic as above applies when C is nodal, or at least has mild singularities if we replace the canonical bundle KC (which we were 1 2 H0( ) defining as WC) by wC, the dualizing sheaf. So, the element a wCe can be viewed as a meromorphic differential on the normalization Ce of C with the property that for any p 2 C, Resp (a · f ) = 0 for any holomorphic f defined in a neighborhood of p. 4 AARON LANDESMAN It’s important to note that the function f above is on the curve C, but the differential a is thought of as on the normalization. Or rather, if p : Ce ! C is 2 H0( ) the map from the normalization, we can view a p∗wCe . Question 3.13. What does wC look like? 3.4.1. At smooth points. First, let’s answer this question at smooth points. i Start with a curve C and a local coordinate x. Then, a = ∑i aix dx. We have the condition that for any n ≥ 1, we must have n−1 0 = Res ax = a−n. Therefore, a−n = 0 for all n ≥ 1, we find a must be regular. Remark 3.14. Hence, in the case C is smooth, we see that we exactly recover KC. 3.4.2. At nodes. Next, let’s answer this question Question 3.13 at nodal points. We start with local coordinates x and y at the two branches of the nodes, with xy = 0. We can write i i a = ∑(aix dx + biy dy). i Then, let’s investigate what the residue condition tells us. We certainly must have Res a = 0. So, a−1 + b−1 = Res a = 0. Also, for any n ≥ 2 n−1 0 = Res a · x = a−n. Similarly, b−n = 0. Therefore, the condition that Resp (a · f ) = 0 tells us a has at worst a simple pole along both branches with opposite residues. Corollary 3.15. We have deg wC = 2g − 2 for a nodal curve. Proof. This is true for a smooth curve. One can then use the explicit descrip- tion of the dualizing sheaf above relating the curve to its normalization to see this statement also holds for nodal curves. Essentially, when we add a node, we increase the genus by 1, and we also increase the degree of wC by 2. Exercise 3.16. Fill in the details of this proof. CURVES NOTES 5 3.5. Constructing sections of a line bundle. Suppose we have a curve C mapping to P2 by f so that f (C) has at worst nodal singularities. Let the image of C be F(x, y, z) = 0 with F homogeneous of degree d. We let G ⊂ f (C) denote the set of nodes of f (C) which are not nodes of C. Let’s now write down sections of wC explicitly in terms of the equation F. On C, we have ¶F ¶F ¶F dx + dy + dz = dF = 0. ¶x ¶y ¶z Therefore, ¶F ¶F ¶F z dx + dy + dz = z(dF) = 0. ¶x ¶y ¶z We also have ¶F ¶F ¶F x + y + z = deg F · F. ¶x ¶y ¶z Therefore, we find ¶F ¶F ¶F ¶F ¶F ¶F z dx + dy + dz = 0 = x + y + z dz. ¶x ¶y ¶z ¶x ¶y ¶z Rearranging, we find y dz − z dy z dy − x dz = ¶F ¶F . ¶x ¶y By symmetry, we have y dz − z dy z dy − x dz x dy − y dx = = ¶F ¶F ¶F . ¶x ¶y ¶z or possibly a different one of the above three expressions, depending on ¶F whether ¶z is nonzero. This produces an expression which is a holomorphic differential. We’ll pick this up from here next class. 4. 9/25/19 We’ll start by recalling the basic setup from last time. We had a nodal curve C with a map f : C ! P2. We called the image curve F(x, y, z) = 0. The nodes of C gave rise to nodes of F, but we may also have additional nodes. We call G the set of additional nodes of F.
Load more

Recommended publications
  • Hyperelliptic Curves with Many Automorphisms
    Hyperelliptic Curves with Many Automorphisms Nicolas M¨uller Richard Pink Department of Mathematics Department of Mathematics ETH Z¨urich ETH Z¨urich 8092 Z¨urich 8092 Z¨urich Switzerland Switzerland [email protected] [email protected] November 17, 2017 To Frans Oort Abstract We determine all complex hyperelliptic curves with many automorphisms and decide which of their jacobians have complex multiplication. arXiv:1711.06599v1 [math.AG] 17 Nov 2017 MSC classification: 14H45 (14H37, 14K22) 1 1 Introduction Let X be a smooth connected projective algebraic curve of genus g > 2 over the field of complex numbers. Following Rauch [17] and Wolfart [21] we say that X has many automorphisms if it cannot be deformed non-trivially together with its automorphism group. Given his life-long interest in special points on moduli spaces, Frans Oort [15, Question 5.18.(1)] asked whether the point in the moduli space of curves associated to a curve X with many automorphisms is special, i.e., whether the jacobian of X has complex multiplication. Here we say that an abelian variety A has complex multiplication over a field K if ◦ EndK(A) contains a commutative, semisimple Q-subalgebra of dimension 2 dim A. (This property is called “sufficiently many complex multiplications” in Chai, Conrad and Oort [6, Def. 1.3.1.2].) Wolfart [22] observed that the jacobian of a curve with many automorphisms does not generally have complex multiplication and answered Oort’s question for all g 6 4. In the present paper we answer Oort’s question for all hyperelliptic curves with many automorphisms.
    [Show full text]
  • Diophantine Problems in Polynomial Theory
    Diophantine Problems in Polynomial Theory by Paul David Lee B.Sc. Mathematics, The University of British Columbia, 2009 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in The College of Graduate Studies (Mathematics) THE UNIVERSITY OF BRITISH COLUMBIA (Okanagan) August 2011 c Paul David Lee 2011 Abstract Algebraic curves and surfaces are playing an increasing role in mod- ern mathematics. From the well known applications to cryptography, to computer vision and manufacturing, studying these curves is a prevalent problem that is appearing more often. With the advancement of computers, dramatic progress has been made in all branches of algebraic computation. In particular, computer algebra software has made it much easier to find rational or integral points on algebraic curves. Computers have also made it easier to obtain rational parametrizations of certain curves and surfaces. Each algebraic curve has an associated genus, essentially a classification, that determines its topological structure. Advancements on methods and theory on curves of genus 0, 1 and 2 have been made in recent years. Curves of genus 0 are the only algebraic curves that you can obtain a rational parametrization for. Curves of genus 1 (also known as elliptic curves) have the property that their rational points have a group structure and thus one can call upon the massive field of group theory to help with their study. Curves of higher genus (such as genus 2) do not have the background and theory that genus 0 and 1 do but recent advancements in theory have rapidly expanded advancements on the topic.
    [Show full text]
  • Determining the Automorphism Group of a Hyperelliptic Curve
    Determining the Automorphism Group of a Hyperelliptic Curve ∗ Tanush Shaska Department of Mathematics University of California at Irvine 103 MSTB, Irvine, CA 92697 ABSTRACT 1. INTRODUCTION In this note we discuss techniques for determining the auto- Let Xg be an algebraic curve of genus g defined over an morphism group of a genus g hyperelliptic curve Xg defined algebraically closed field k of characteristic zero. We denote over an algebraically closed field k of characteristic zero. by Aut(Xg) the group of analytic (equivalently, algebraic) The first technique uses the classical GL2(k)-invariants of automorphisms of Xg. Then Aut(Xg) acts on the finite set binary forms. This is a practical method for curves of small of Weierstrass points of Xg. This action is faithful unless genus, but has limitations as the genus increases, due to the Xg is hyperelliptic, in which case its kernel is the group of fact that such invariants are not known for large genus. order 2 containing the hyperelliptic involution of Xg. Thus The second approach, which uses dihedral invariants of in any case, Aut(Xg) is a finite group. This was first proved hyperelliptic curves, is a very convenient method and works by Schwartz. In 1893 Hurwitz discovered what is now called well in all genera. First we define the normal decomposition the Riemann-Hurwitz formula. From this he derived that of a hyperelliptic curve with extra automorphisms. Then dihedral invariants are defined in terms of the coefficients of |Aut(Xg)| ≤ 84 (g − 1) this normal decomposition. We define such invariants inde- which is known as the Hurwitz bound.
    [Show full text]
  • Geometry of Algebraic Curves
    Geometry of Algebraic Curves Fall 2011 Course taught by Joe Harris Notes by Atanas Atanasov One Oxford Street, Cambridge, MA 02138 E-mail address: [email protected] Contents Lecture 1. September 2, 2011 6 Lecture 2. September 7, 2011 10 2.1. Riemann surfaces associated to a polynomial 10 2.2. The degree of KX and Riemann-Hurwitz 13 2.3. Maps into projective space 15 2.4. An amusing fact 16 Lecture 3. September 9, 2011 17 3.1. Embedding Riemann surfaces in projective space 17 3.2. Geometric Riemann-Roch 17 3.3. Adjunction 18 Lecture 4. September 12, 2011 21 4.1. A change of viewpoint 21 4.2. The Brill-Noether problem 21 Lecture 5. September 16, 2011 25 5.1. Remark on a homework problem 25 5.2. Abel's Theorem 25 5.3. Examples and applications 27 Lecture 6. September 21, 2011 30 6.1. The canonical divisor on a smooth plane curve 30 6.2. More general divisors on smooth plane curves 31 6.3. The canonical divisor on a nodal plane curve 32 6.4. More general divisors on nodal plane curves 33 Lecture 7. September 23, 2011 35 7.1. More on divisors 35 7.2. Riemann-Roch, finally 36 7.3. Fun applications 37 7.4. Sheaf cohomology 37 Lecture 8. September 28, 2011 40 8.1. Examples of low genus 40 8.2. Hyperelliptic curves 40 8.3. Low genus examples 42 Lecture 9. September 30, 2011 44 9.1. Automorphisms of genus 0 an 1 curves 44 9.2.
    [Show full text]
  • Symmetry Types of Hyperelliptic Riemann Surfaces
    MEMOIRES´ DE LA SMF 86 SYMMETRY TYPES OF HYPERELLIPTIC RIEMANN SURFACES Emilio Bujalance Francisco-Javier Cirre J.-M. Gamboa Grzegorz Gromadzki Soci´et´eMath´ematique de France 2001 Publi´eavecleconcoursduCentreNationaldelaRechercheScientifique 2000 Mathematics Subject Classification.— Key words and phrases.— Abstract.— X τ : X X → Aut±X X g ! 2 Résumé (Types de symétrie des surfaces de Riemann hyperelliptiques) X τ : X X → Aut±X X g ! 2 ! CONTENTS Introduction................................................................... 1 1. Preliminaries............................................................... 7 1.1. Klein surfaces and NEC groups.. 7 1.2. Symmetric Riemann surfaces. 13 1.3. Hyperelliptic real algebraic curves........................................ 15 2. Automorphism groups of symmetric hyperelliptic Riemann surfaces.. 21 2.1. On signatures of the automorphism groups.. 21 2.2. Number of real forms. 26 2.3. Automorphism groups of symmetric hyperelliptic Riemann surfaces. 43 3. Symmetry types of hyperelliptic Riemann surfaces.................... 49 3.1. Symmetry types of hyperelliptic algebraic curves of class I. 50 3.2. Symmetry types of hyperelliptic algebraic curves of class II. ............. 65 3.3. Symmetry types of hyperelliptic algebraic curves of class III. ........... 72 3.4. Symmetry types of hyperelliptic algebraic curves of class IV. ............. 79 3.5. Symmetry types of hyperelliptic algebraic curves of class V. ............. 92 3.6. Symmetry types of hyperelliptic algebraic curves of class VI. ............. 97 3.7. Symmetry types of hyperelliptic algebraic curves of class VII. ............102 3.8. Symmetry types of hyperelliptic algebraic curves of class VIII............109 3.9. Symmetry types of hyperelliptic algebraic curves of class IX. .............115 3.10. Symmetry types of hyperelliptic algebraic curves of class X. ..........117 Bibliography...................................................................119 INTRODUCTION Let X be a compact Riemann surface.
    [Show full text]
  • Log Minimal Model Program for the Moduli Space of Stable Curves: the Second Flip
    LOG MINIMAL MODEL PROGRAM FOR THE MODULI SPACE OF STABLE CURVES: THE SECOND FLIP JAROD ALPER, MAKSYM FEDORCHUK, DAVID ISHII SMYTH, AND FREDERICK VAN DER WYCK Abstract. We prove an existence theorem for good moduli spaces, and use it to construct the second flip in the log minimal model program for M g. In fact, our methods give a uniform self-contained construction of the first three steps of the log minimal model program for M g and M g;n. Contents 1. Introduction 2 Outline of the paper 4 Acknowledgments 5 2. α-stability 6 2.1. Definition of α-stability6 2.2. Deformation openness 10 2.3. Properties of α-stability 16 2.4. αc-closed curves 18 2.5. Combinatorial type of an αc-closed curve 22 3. Local description of the flips 27 3.1. Local quotient presentations 27 3.2. Preliminary facts about local VGIT 30 3.3. Deformation theory of αc-closed curves 32 3.4. Local VGIT chambers for an αc-closed curve 40 4. Existence of good moduli spaces 45 4.1. General existence results 45 4.2. Application to Mg;n(α) 54 5. Projectivity of the good moduli spaces 64 5.1. Main positivity result 67 5.2. Degenerations and simultaneous normalization 69 5.3. Preliminary positivity results 74 5.4. Proof of Theorem 5.5(a) 79 5.5. Proof of Theorem 5.5(b) 80 Appendix A. 89 References 92 1 2 ALPER, FEDORCHUK, SMYTH, AND VAN DER WYCK 1. Introduction In an effort to understand the canonical model of M g, Hassett and Keel introduced the log minimal model program (LMMP) for M .
    [Show full text]
  • Ordinary Plane Models and Completely Split Divisors
    Ordinary plane models and completely split divisors Claus Diem and Sebastian Kochinke March 21, 2019 Abstract Let C be a smooth, non-hyperelliptic curve over an algebraically closed field of genus g ≥ 4. We show that the projection from the canonical model of C through (g −3) generic points on C is a birational morphism to a plane curve which has only finitely many non-ordinary tangents, that is, flex- or bitangents. For smooth, non-hyperelliptic curves of a fixed genus g ≥ 4 over finite fields, we show that the proba- bility that an effective divisor of degree (g−3) defines such an \ordinary (birational) plane model" converges to 1. This result has an application to the solution of the discrete loga- rithm problem for smooth, non-hyperelliptic curves curves of a fixed genus g over finite fields Fq: By first changing the representation to such an ordinary plane model and then using an algorithm by the first 2 ~ 2− g−1 author, the problem can be solved in an expected time of O(q ). Another consequence is that for smooth, non-hyperelliptic curves of a fixed genus g over finite fields Fq, the number of completely split 1 q−1 divisors in the canonical system is ∼ (2g+2)! ⋅ q . 1 Introduction Let us consider the discrete logarithm problem in the degree-0 Picard groups of smooth curves of a fixed genus g. In [Die11] it is shown that this problem can be solved in an expected time of 2− 2 ~ g Oq ; here and in the following the phrase \expected time" refers to an internal randomization of the algorithm.
    [Show full text]
  • 1 Riemann Surfaces - Sachi Hashimoto
    1 Riemann Surfaces - Sachi Hashimoto 1.1 Riemann Hurwitz Formula We need the following fact about the genus: Exercise 1 (Unimportant). The genus is an invariant which is independent of the triangu- lation. Thus we can speak of it as an invariant of the surface, or of the Euler characteristic χ(X) = 2 − 2g. This can be proven by showing that the genus is the dimension of holomorphic one forms on a Riemann surface. Motivation: Suppose we have some hyperelliptic curve C : y2 = (x+1)(x−1)(x+2)(x−2) and we want to determine the topology of the solution set. For almost every x0 2 C we can find two values of y 2 C such that (x0; y) is a solution. However, when x = ±1; ±2 there is only one y-value, y = 0, which satisfies this equation. There is a nice way to do this with branch cuts{the square root function is well defined everywhere as a two valued functioned except at these points where we have a portal between the \positive" and the \negative" world. Here it is best to draw some pictures, so I will omit this part from the typed notes. But this is not very systematic, so let me say a few words about our eventual goal. What we seem to be doing is projecting our curve to the x-coordinate and then considering what this generically degree 2 map does on special values. The hope is that we can extract from this some topological data: because the sphere is a known quantity, with genus 0, and the hyperelliptic curve is our unknown, quantity, our goal is to leverage the knowns and unknowns.
    [Show full text]
  • Geometry of Algebraic Curves
    Geometry of Algebraic Curves Lectures delivered by Joe Harris Notes by Akhil Mathew Fall 2011, Harvard Contents Lecture 1 9/2 x1 Introduction 5 x2 Topics 5 x3 Basics 6 x4 Homework 11 Lecture 2 9/7 x1 Riemann surfaces associated to a polynomial 11 x2 IOUs from last time: the degree of KX , the Riemann-Hurwitz relation 13 x3 Maps to projective space 15 x4 Trefoils 16 Lecture 3 9/9 x1 The criterion for very ampleness 17 x2 Hyperelliptic curves 18 x3 Properties of projective varieties 19 x4 The adjunction formula 20 x5 Starting the course proper 21 Lecture 4 9/12 x1 Motivation 23 x2 A really horrible answer 24 x3 Plane curves birational to a given curve 25 x4 Statement of the result 26 Lecture 5 9/16 x1 Homework 27 x2 Abel's theorem 27 x3 Consequences of Abel's theorem 29 x4 Curves of genus one 31 x5 Genus two, beginnings 32 Lecture 6 9/21 x1 Differentials on smooth plane curves 34 x2 The more general problem 36 x3 Differentials on general curves 37 x4 Finding L(D) on a general curve 39 Lecture 7 9/23 x1 More on L(D) 40 x2 Riemann-Roch 41 x3 Sheaf cohomology 43 Lecture 8 9/28 x1 Divisors for g = 3; hyperelliptic curves 46 x2 g = 4 48 x3 g = 5 50 1 Lecture 9 9/30 x1 Low genus examples 51 x2 The Hurwitz bound 52 2.1 Step 1 . 53 2.2 Step 10 ................................. 54 2.3 Step 100 ................................ 54 2.4 Step 2 .
    [Show full text]
  • The Case for Superelliptic Curves
    The case for superelliptic curves L. BESHAJ a T. SHASKA b E. ZHUPA c a Department of Mathematics, Oakland University, Rochester, MI, USA; E-mail: [email protected] b Department of Mathematics, Oakland University, Rochester, MI, USA; E-mail: [email protected] c Department of Computer Science, University of Information Science and Technology ”St. Paul the Apostle”, Ohrid, Republic of Macedonia; E-mail: [email protected] Abstract. There is a natural question to ask whether the rich mathemat- ical theory of the hyperelliptic curves can be extended to all superellip- tic curves. Moreover, one wonders if all of the applications of hyperellip- tic curves such as cryptography, mathematical physics, quantum com- putation, diophantine geometry, etc can carry over to the superelliptic curves. In this short paper we make the case that the superelliptic curves are exactly the curves that one should study Keywords. hyperelliptic curves, superelliptic curves, moduli space 1. Introduction This lecture is intended to be a motivation for the study of superelliptic curves arXiv:1502.07249v1 [math.AG] 25 Feb 2015 which were the main focus of the NATO Advanced Study Institute held in Ohrid, Macedonia in the Summer 2014. While the scope of interesting mathematical problems related to superelliptic curves is very broad and the applications include different areas of sciences, in this paper we focus on few of the arithmetic problems related to the moduli space of curves, automorphisms groups, minimal models of curves, and rational points on curves with the intention to emphasize the methods of extending the knowledge of hyperelliptic curves to all superelliptic curves.
    [Show full text]
  • SOME OPEN PROBLEMS in COMPUTATIONAL ALGEBRAIC GEOMETRY Contents 1. Introduction 298 Part 1. Algebraic Curves 299 2. Genus 2 Curv
    ALBANIAN JOURNAL OF MATHEMATICS Volume 1, Number 4, Pages 297{319 ISSN 1930-1235: (2007) SOME OPEN PROBLEMS IN COMPUTATIONAL ALGEBRAIC GEOMETRY TANUSH SHASKA To my wonderful children Rachel, Adrianna, Eva, and Besianna. Abstract. The development of computational techniques in the last decade has made possible to attack some classical problems of algebraic geometry from a computational viewpoint. In this survey, we briefly describe some open problems of computational algebraic geometry which can be approached from such viewpoint. Some of the problems we discuss are the decomposition of Jacobians of genus two curves, automorphisms groups of algebraic curves and the corresponding loci in the moduli space of algebraic curves Mg, inclusions among such loci, decomposition of Jacobians of algebraic curves with automor- phisms, invariants of binary forms and the hyperelliptic moduli, theta functions of curves with automorphisms, etc. We decompose Jacobians of genus 3 curves with automorphisms and determine the inclusions among the loci for algebraic curves with automorphisms of genus 3 and 4. Contents 1. Introduction 298 Part 1. Algebraic curves 299 2. Genus 2 curves with (n; n)-split Jacobian 300 2.1. Covers of odd degree 301 2.2. Covers of even degree 301 3. The automorphism group of algebraic curves 303 3.1. Inclusion among the loci of curves with prescribed automorphism group 303 3.2. Hurwitz curves 309 4. The automorphism groups of algebraic curves in positive characteristic 309 4.1. Cyclic curves in characteristic p > 0 310 5. On the decomposition of Jacobians of algebraic curves with automorphisms 311 5.1. Decomposing Jacobians of genus three algebraic curves with automorphisms 312 6.
    [Show full text]
  • HITCHIN SYSTEMS on HYPERELLIPTIC CURVES 2 of This Series the Separation of Variables Is Carried out in [2]
    HITCHIN SYSTEMS ON HYPERELLIPTIC CURVES P.I. BORISOVA, O.K. SHEINMAN Abstract. We describe a class of spectral curves and find explicit formulas for Dar- boux coordinates for hyperelliptic Hitchin systems corresponding to classical simple Lie groups. We consider in detail the systems with classical rank 2 gauge groups on genus 2 curves. Contents 1. Introduction 1 2. Hitchin systems on hyperelliptic curves 2 2.1. Hitchin systems 2 2.2. Spectral curves of hyperelliptic Hitchin systems 3 3. Separation of variables for hyperelliptic Hitchin systems 4 3.1. Separating variables. Symplectic and Poisson structure 4 3.2. The form of Hamiltonians in separating variables 5 3.3. Darboux coordinates 6 3.4. Action-angle coordinates 7 4. Systems of rank 1 and 2 on hyperelliptic curves of genus 2 8 4.1. Hitchin system for the Lie algebra sl(2) on a hyperelliptic curve of genus 2 8 4.2. Hitchin system for the Lie algebra so(4) on a hyperelliptic curve of genus 2 9 4.3. Hitchin system for the Lie algebra sp(4) on a hyperelliptic curve of genus 2 12 4.4. Hitchin system for the Lie algebra so(5) on a hyperelliptic curve of genus 2 13 5. Discussion 13 References 15 arXiv:1912.06849v1 [math-ph] 14 Dec 2019 1. Introduction In the vast literature on Hitchin systems there are not so many works devoted to the fundamental question of separation of variables for them. With this relation, we can only mention [7, 11, 9]. The first one treats in detail the system on a genus 2 curve with SL(2) as a gauge group, and in the two others some general approaches to the problem are discussed.
    [Show full text]