11.2 Riemann surfaces

Riemann surfaces are like magical maps for complex functions. They turn tricky multivalued functions into smooth, single-valued ones by creating extra dimensions. This lets us explore these functions without getting lost in their twists and turns.

By connecting different "sheets" of the complex plane, Riemann surfaces give us a clear picture of how functions behave. They're key to understanding complex analysis, helping us navigate the intricate world of multivalued functions with ease.

Riemann Surfaces: Concept and Construction

Definition and Local Structure

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• A Riemann surface is a one-dimensional complex manifold, a surface equipped with a complex structure that makes it locally similar to the complex plane
• Riemann surfaces are locally homeomorphic to the complex plane, meaning that small neighborhoods on the surface can be mapped to open sets in the complex plane using a continuous and invertible function
• Every point on a Riemann surface has a local coordinate chart that maps a neighborhood of the point to an open set in the complex plane, preserving the complex structure

Construction Process

• Riemann surfaces are constructed by gluing together copies of the complex plane, known as sheets, along branch cuts in a way that preserves the complex structure
• The construction of a Riemann surface involves identifying and connecting the branches of a multivalued function, ensuring that the function is single-valued and analytic on the resulting surface
• The process of constructing a Riemann surface can be visualized as taking a multivalued function defined on the complex plane and "unfolding" it into a single-valued function on a higher-dimensional surface
• The number of sheets in a Riemann surface corresponds to the number of branches of the multivalued function (square root function has two sheets, logarithm function has infinite sheets)
• Branch points on the complex plane are the points where the sheets of the Riemann surface are connected, and the function is not single-valued (branch point at z = 0 for square root and logarithm functions)

Properties and Structure of Riemann Surfaces

Topological Properties

• Riemann surfaces are orientable, meaning that a consistent notion of "clockwise" and "counterclockwise" can be defined on the surface
• The genus of a Riemann surface is a topological invariant that measures the number of "holes" or "handles" in the surface
  • The genus is related to the degree of the multivalued function and the number of branch points
  • A sphere has genus 0, a torus has genus 1, and a double torus has genus 2
• Compact Riemann surfaces, those that are closed and bounded, have a well-defined genus and can be classified up to homeomorphism based on their genus
• Non-compact Riemann surfaces, such as the complex plane itself or the Riemann surface of the logarithm function, have infinite genus and cannot be classified in the same way as compact surfaces

Analytic Structure

• The complex structure on a Riemann surface allows for the definition of analytic functions on the surface
• Analytic functions on a Riemann surface are locally given by power series expansions in the local coordinate charts
• The global analytic structure of a Riemann surface is determined by the transition functions between the local coordinate charts, which must be analytic functions themselves
• Meromorphic functions on a Riemann surface are analytic functions that may have poles, points where the function approaches infinity
• The study of meromorphic functions on Riemann surfaces is a central topic in complex analysis and algebraic geometry

Riemann Surfaces and Multivalued Functions

Resolving Multivaluedness

• Multivalued functions are functions that assign multiple values to each point in their domain, such as the square root function ($f(z) = \sqrt{z}$) or the logarithm function ($f(z) = \log(z)$)
• Riemann surfaces provide a way to "resolve" the multivalued nature of these functions by creating a higher-dimensional surface on which the function becomes single-valued and analytic
• Each branch of a multivalued function corresponds to a sheet on the Riemann surface, and the branch cuts on the complex plane correspond to the connections between the sheets
• The Riemann surface of a multivalued function can be thought of as the natural domain of the function, where it behaves as a single-valued and analytic function

Function Properties and Surface Structure

• The properties of the Riemann surface, such as its genus and the number of sheets, are determined by the properties of the multivalued function, such as its degree and the location of its branch points
• For example, the Riemann surface of the square root function has two sheets and genus 0, while the Riemann surface of the logarithm function has infinitely many sheets and infinite genus
• The branch points of a multivalued function determine the connectivity of the sheets on the Riemann surface and the structure of the branch cuts
• Understanding the relationship between the properties of a multivalued function and its Riemann surface is crucial for studying the function's behavior and for solving problems involving integration and path-dependence

Constructing Riemann Surfaces for Multivalued Functions

Square Root and Logarithm Functions

• For the square root function, $f(z) = \sqrt{z}$, there is a single branch point at $z = 0$, and the branch cut is typically chosen along the negative real axis
  • The Riemann surface consists of two sheets connected along the branch cut
  • One sheet corresponds to the principal square root, while the other corresponds to the negative of the principal square root
• For the logarithm function, $f(z) = \log(z)$, there is a single branch point at $z = 0$, and the branch cut is typically chosen along the negative real axis
  • The Riemann surface consists of an infinite number of sheets, each corresponding to a different branch of the logarithm
  • Each sheet is connected to the next sheet by adding or subtracting $2\pi i$, representing the periodicity of the exponential function

Complex Power and More General Functions

• For the complex power function, $f(z) = z^{\alpha}$, where $\alpha$ is a non-integer, there is a branch point at $z = 0$, and the branch cut is typically chosen along the positive real axis
  • The Riemann surface consists of a finite number of sheets determined by the value of $\alpha$
  • For example, if $\alpha = 1/n$, the Riemann surface will have $n$ sheets, each corresponding to a different $n$-th root of unity
• More complex multivalued functions may have multiple branch points and require more intricate branch cuts and sheet structures in their Riemann surfaces
  • The choice of branch cuts is not unique, and different choices can lead to homeomorphic but distinct Riemann surfaces for the same multivalued function
  • Constructing Riemann surfaces for general multivalued functions involves careful analysis of the function's branch points, branch cuts, and the connectivity of the sheets
• Understanding the construction of Riemann surfaces for specific multivalued functions provides insight into their properties and behavior, and serves as a foundation for more advanced topics in complex analysis and algebraic geometry