some questions on Riemann surface - MathOverflow

some questions on Riemann surface

2011-02-10T10:56:21Z

There are several puzzling questions on Riemann surface for me: Q.1 Definition of Riemann surface can be given in at least two ways: Def.1) it is a complex one dimensional manifold; Def.2) for each $a\in \mathbb{C}$, consider collection of germs at $a$, of analytic functions, and give a topology on it. Are these really equivalent definitions? or Def.2 is more general than Def.1?

Q.2 When we say a group $G$ is an automorphism group of a compact Riemann surface, how is the action? (for ex. what is description of action of of PSL(2,7) on a genus 3 Riemann surface? In the book of Thomas Breuer, I couldn't see any description of action of a group on a Riemann surface; he has given computational methods to investigate groups.)

Q.3 The automorphisms of a compact Riemann surface can always be lifted to universal cover?

Q.4 If a group $G$ acts on a compact Riemann surface $X_g$, of genus $g$, then $X_g/G$ is also a compact Riemann surface of some genus $h$ and $g,h$ are related by Riemann-Hurwitz formula. Can anyone suggest some good reference for this relation? (here, I would like to see this Riemann Hurwitz relation topologically; many books describe it using algebraic geometry techniques).

(I went through many books on Riemann surface for these questions; but not understood many things)

Answer by Charles Matthews for some questions on Riemann surface

2011-02-10T12:01:16Z

Q1: Use the manifold definition, going back to Weyl. The other definition comes out of the theory of analytic continuation. (And is somewhat puzzling historically - I'm not quite sure how the Poincaré-Volterra theorem fits in, but these days you'd probably want to read this material in terms of sheaf theory, to which it was one of the inputs.)

Q2: G acts on the field of meromorphic functions, is one way to look at it. These are holomorphic mappings of the surface to itself, described by some algebraic mappings in fact.

Q3: I think so, by "abstract nonsense".

Q4: The quotient is to be treated carefully, since quotients of manifolds are not always manifolds. But in terms of the function field this can be seen as Galois theory, and X is a ramified (usually) covering of the quotient curve. The topological explanation of the Euler characteristic in the Riemann-Hurwitz formula is intuitively clear: just look at what happens under the k-th power map on the unit complex disc, in terms of a simple triangulation, to see how ramification affects coverings.

Answer by Alexandre Eremenko for some questions on Riemann surface

2012-08-04T12:18:25Z

Q1. There are two DIFFERENT notions of Riemann surface in the literature.

a) One-dimensional complex analytic manifold (coming from the book of Weyl).

b) Riemann surface "spread over the plane (or over the Riemann sphere)". Your second definition, the set of germs with an appropriate topology on it, formalizes this second notion.

Older books seem to understand Riemann surfaces in the sense of the second definition. Sometimes a) was called an "abstract Riemann surface" in these books.

For most mathematicians with modern training the "Riemann surface of log z" and the "Riemann surface of arccos z" are meaningless expressions because these are the same as the plane, in the sense of definition a).

The formal relation between a) and b) is the following. "A Riemann surface spread over the plane" is a pair (S,f), where S is an abstract Riemann surface and f is a holomorphic function from S to C. (If f is meromorphic, we have a Riemann surface spread over the sphere.)

Here is another way to say this. Let S be a Riemann surface in the sense a). It has a set of charts $\phi_j: U_j\to D_j$ from the elements of an open covering U to discs D in the plane. The correspoddence maps $\phi_k\circ\phi_j^{-1}$ on $D_j\cap D_k$ must be conformal.

Now let us require that these correspondence maps be IDENTITY maps of $D_j\cap D_k$. Then we obtain notion b). This is an additional structure on a Riemann surface in the sense a) which is sometimes called a flat structure.

If you look carefully (say, on the example of arccos) you will see that the two definitions of a Riemann surface in the sense b) that I gave are not exactly equivalent. More about this in my survey "Geometric theory of meromorphic functions", and in the preprint of Biswas and Perez Marco, Log Riemann Surfaces.