### Lecture Description

Goals: - To see how the topological quotient of the universal covering of a space by the deck transformation group (which is isomorphic to the fundamental group of the space) gives back the space - In particular, the topological universal covering of a Riemann surface (which inherits a unique Riemann surface structure as shown in the previous lecture) modulo (or quotiented by or divided by) the fundamental group gives back the Riemann surface - To see that nontrivial deck transformations are fixed-point free - To see why any Riemann surface with universal covering the Riemann sphere is isomorphic to the Riemann sphere itself - To get a characterization of discrete subgroups of the additive group of complex numbers - To use the above characterisation to deduce that a Riemann surface with universal covering the plane has to be isomorphic to either the plane itself, or to a complex cylinder, or to a complex torus Topics: Holomorphic covering, holomorphic universal covering, group action on a topological space, orbit of a group action, equivalence relation defined by a group action, quotient by a group, topological quotient, quotient topology, quotient map, transitive action, deck transformation, open map, Riemann sphere, one-point compactification, stereographic projection, Möbius transformation, unique lifting property, group of translations, admissible neighborhood, module, submodule, subgroup, discrete submodule, discrete subgroup

### Course Index

- The Idea of a Riemann Surface
- Simple Examples of Riemann Surfaces
- Maximal Atlases and Holomorphic Maps of Riemann Surfaces
- Riemann Surface Structure on a Cylinder
- Riemann Surface Structure on a Torus
- Riemann Surface Structures on Cylinders and Tori via Covering Spaces
- Möbius Transformations Make up Fundamental Groups of Riemann Surfaces
- Homotopy and the First Fundamental Group
- A First Classification of Riemann Surfaces
- The Importance of the Path-lifting Property
- Fundamental groups as Fibres of the Universal covering Space
- The Monodromy Action
- The Universal covering as a Hausdorff Topological Space
- The Construction of the Universal Covering Map
- Universality of the Universal Covering
- The Fundamental Group of the base as the Deck Transformation Group
- The Riemann Surface Structure on the Topological Covering of a Riemann Surface
- Riemann Surfaces with Universal Covering the Plane or the Sphere
- Classifying Complex Cylinders Riemann Surfaces
- Möbius Transformations with a Single Fixed Point
- Möbius Transformations with Two Fixed Points
- Torsion-freeness of the Fundamental Group of a Riemann Surface
- Characterizing Riemann Surface Structures on Quotients of the Upper Half
- Classifying Annuli up to Holomorphic Isomorphism
- Orbits of the Integral Unimodular Group in the Upper Half-Plane
- Galois Coverings are precisely Quotients by Properly Discontinuous Free Actions
- Local Actions at the Region of Discontinuity of a Kleinian Subgroup
- Quotients by Kleinian Subgroups give rise to Riemann Surfaces
- The Unimodular Group is Kleinian
- The Necessity of Elliptic Functions for the Classification of Complex Tori
- The Uniqueness Property of the Weierstrass Phe-function
- The First Order Degree Two Cubic Ordinary Differential Equation satisfied by the Weierstrass Phe-function
- The Values of the Weierstrass Phe function at the Zeros of its Derivative
- The Construction of a Modular Form of Weight Two on the Upper Half-Plane
- The Fundamental Functional Equations satisfied by the Modular Form of Weight
- The Weight Two Modular Form assumes Real Values on the Imaginary Axis
- The Weight Two Modular Form Vanishes at Infinity
- The Weight Two Modular Form Decays Exponentially in a Neighbourhood of Infinity
- Suitable Restriction of the Weight Two Modular Form is a Holomorphic Conformal Isomorphism onto the Upper Half-Plane
- The J-Invariant of a Complex Torus (or) of an Algebraic Elliptic Curve
- Fundamental Region in the Upper Half-Plane for the Elliptic Modular J-Invariant
- The Fundamental Region in the Upper Half-Plane for the Unimodular Group
- A Region in the Upper Half-Plane Meeting Each Unimodular Orbit Exactly Once
- Moduli of Elliptic Curves
- Punctured Complex Tori are Elliptic Algebraic Affine Plane
- The Natural Riemann Surface Structure on an Algebraic Affine Nonsingular Plane Curve
- Complex Projective 2-Space as a Compact Complex Manifold of Dimension Two
- Complex Tori are the same as Elliptic Algebraic Projective Curves

### Course Description

The subject of algebraic curves (equivalently compact Riemann surfaces) has its origins going back to the work of Riemann, Abel, Jacobi, Noether, Weierstrass, Clifford and Teichmueller. It continues to be a source for several hot areas of current research. Its development requires ideas from diverse areas such as analysis, PDE, complex and real differential geometry, algebra---especially commutative algebra and Galois theory, homological algebra, number theory, topology and manifold theory. The course begins by introducing the notion of a Riemann surface followed by examples. Then the classification of Riemann surfaces is achieved on the basis of the fundamental group by the use of covering space theory and uniformisation. This reduces the study of Riemann surfaces to that of subgroups of Moebius transformations. The case of compact Riemann surfaces of genus 1, namely elliptic curves, is treated in detail. The algebraic nature of elliptic curves and a complex analytic construction of the moduli space of elliptic curves is given.