# Riemann-Roch and Serre duality on compact Riemann surfaces

In this post, I will rely on this last post for results and notations, for example will also mean a compact Riemann surface. We will note the canonical bundle of , i.e. the bundle with sheaf of sections the holomorphic 1-forms on . We will also note the dimension of by which we really mean the j’th cohomology group of in the sheaf of holomorphic sections of . I’ll also write to mean where is the line bundle associated with the divisor , and sometimes the line bundle might get written accordingly. We will sometimes also suppress the and simply write for . Let us first state the Riemann-Roch theorem in the form that we aim to get to:

Theorem (Riemann-Roch):For any holomorphic line bundle , the following relation holds:.

We will freely use the fact that any line bundle on a compact Riemann surface actually comes from a divisor, which can be seen by Jacobi’s inversion theorem, and define the degree of simply as the degree of its associated divisor. To be more precise, we will do the proof for line bundles coming from generic divisors with the ‘s distinct. We will first prove that

which is relatively easy. The hard part will be

Theorem (Serre duality): There is a natural coupling which induces an isomorphism

In particular, this tells us that and Riemann-Roch follows.

**Proof of the first part:** We show the first part by induction on the degree of If , i.e. if is trivial, then by the maximum principle and . So

and we are done with the base case. Now, suppose that we can represent as for a divisor with the plus or minus sign in accordance with the sign of , and with all ‘s distinct.

Suppose by induction that the statement is true for . Then it suffices to show it is true for the line bundles associated to and for some divisor of degree not containing . In the case of positive degree, we use the short exact sequence

.

Note that this short exact sequence was also considered in my last post with trivial . Here we do the usual identification between sections with meromorphic functions such that and the first arrow corresponds to the inclusion of families of these spaces of meromorphic functions. We get a long exact sequence

hence adding alternating dimensions we find

.

Using our induction assumption, i.e. that , we deduce that

which is what we wanted.

For the second case, when , we use another but related short exact sequence:

.

A similar argument shows that

and we are done.

QED

Before attacking Serre duality, recall that for any line bundle there is an operator

where is the sheaf of smooth -forms with values in . It is the unique such operator satisfying Leibniz rule

and such that

is exact. It is called the **del-bar operator associated to **. Analogously to last post, we get a **Dolbeault isomorphism**

.

The identifications we discovered at the end of last post still hold in this context, with the necessary adjustments. Given two line bundles , this isomorphism permits us to define a **pairing**

by setting

.

In particular, with and , we get **Serre’s pairing**

where the last isomorphism is (which follows from the same argument as for ) composed with integration .

Theorem (Serre duality): This pairing is non-degenerate.

**Proof:** We will proceed by induction as in the proof of Riemann-Roch. We say that satisfies Serre duality if Serre’s pairing induces an isomorphism

and .

Note that satisfies Serre duality if and only if does.

The base case is the statement that the pairing given by is non-degenerate, which is just the isomorphism given by integration coupled with the fact that is injective.

Suppose now by induction that all line bundles with satisfy Serre’s duality. Like in Riemann-Roch, it suffices to show that and satisfy Serre duality (where with not containing ). We will now identify with meromorphic sections of with at worst a simple pole at . We thus get the short exact sequence

where the last arrow is (recall that the residue of a meromorphic function at is naturally an element of the holomorphic tangent space , as discussed in my last post and under our identifications, ). We obtain in this way our **first exact sequence**

.

Recall from last post that if for , then for an extension of in the neighborhood of .

Similarly, we see elements of as holomorphic sections of vanishing at , and we have the following short exact sequence of sheaves:

where now the last arrow is simply . This gives our **second exact sequence**

.

Dualizing the first exact sequence, we get the following diagram:

where the vertical arrows are given by Serre’s pairing. By the **Five Lemma **and by using our induction hypothesis, it suffices to show that this diagram is commutative. The only difficult parts are obviously the two squares in the middle.

Actually, to be exactly precise, we need to consider instead of just and similarly .

For **the third square**, consider (so is a holomorphic 1-form and a section of ). We need to see that for all , we have

.

But we have seen in last post that if near , then after identifying with , we have . Thus if , we find

which, since is supported near , by using Stokes theorem and Cauchy’s formula, is equal to

for a small circle around . But , so this shows the third square commutes.

**The fourth square** is similar. Take . Then unwinding the definition of , we see that

if in some coordinates around . Then under the Dolbeault isomorphism, this class corresponds to and it acts on by

where , for and . Like for the third square, this integral can be rewritten

for a small loop around and the residue of in the coordinate. But this is exactly what we needed since

.