# Meromorphic Functions on Riemann surfaces

• lavinia
In summary, Felix Klein discusses methods for constructing divergence-free irrotational flows on compact Riemann surfaces, such as the torus, in his pamphlet "On Riemann's Theory of Algebraic Functions and their Integrals." One approach is to place two oppositely charged electric poles on the surface, creating an electric field with two singularities at the poles. One question raised is how to ensure that the field does not circulate around one of the handles of the surface, such as the meridian on the torus. If the field does not circulate, it can be simplified by combining the two singularities into a meromorphic function. However, if the field does circulate, the potential will be multi-val
lavinia
Gold Member
In Felix's Klein's pamphlet, "On Riemann's Theory of Algebraic Functions and their Integrals" he describes ways to construct divergence free irrotational flows on a compact Riemann surfaces such as the torus.

One method is simply to cover the surface with a conducting material and place two oppositely charged electric poles - using a battery - at two points on the surface. The potential difference at these poles will create an electric field - which may be viewed as a flow.

The field will have two oppositely oriented logarithmic singularities at the poles and will be regular (finite) everywhere else.

My question is: How does one know that the electric field will not circulate around one of the handles of the surface? For instance on the torus, how does one know that it will not circulate around the meridian? By this I mean, how does one know that the integral of the field along a closed homotopically non-trivial loop will be zero? Or is this not true?

If it is true then one can coalesce the two logarithmic singularities to get a new field whose potential is a meromorphic function. But if the field circulates around one of the non-trivial loops its potential will be multi-valued.

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## 1. What is a meromorphic function on a Riemann surface?

A meromorphic function on a Riemann surface is a complex-valued function that is defined and holomorphic (analytic) on all points of the surface except for a set of isolated points where it has poles. These poles can be thought of as "singularities" of the function, where it is not well-defined or behaves in a non-analytic way.

## 2. What is the difference between a meromorphic and holomorphic function on a Riemann surface?

A holomorphic function is defined and analytic on all points of a Riemann surface, while a meromorphic function may have isolated points where it is not defined or not analytic. Essentially, a meromorphic function is a more general class of functions that includes holomorphic functions.

## 3. How are Riemann surfaces related to meromorphic functions?

Riemann surfaces are closely tied to meromorphic functions, as they provide the geometric framework for understanding these functions. A Riemann surface is a complex manifold that allows us to extend the concept of a function to non-holomorphic points, such as poles, where the function may not be well-defined. Riemann surfaces also help us visualize the behavior of meromorphic functions in a more intuitive way.

## 4. What are some examples of meromorphic functions on Riemann surfaces?

Some common examples of meromorphic functions on Riemann surfaces include rational functions, such as f(z) = 1/z, which has a pole at z = 0. Other examples include logarithmic functions, such as f(z) = log(z), which has a pole at z = 0 and is defined and holomorphic on all other points.

## 5. How are meromorphic functions used in mathematics?

Meromorphic functions have many important applications in mathematics, particularly in complex analysis and algebraic geometry. They are essential in the study of Riemann surfaces, as well as in the development of tools and techniques for solving complex analytic problems. Meromorphic functions also have connections to number theory, topology, and physics, making them a crucial topic in many areas of mathematics.

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