# Calculating Sheaf Cohomology of Canonical Bundle over a Curve

I'm trying to solve the following problem.

Let $$C$$ be a smooth projective curve and let $$\Omega^1_C$$ be the canonical bundle. Show that there is a canonical isomorphism, $$H^1(C, \Omega^1_C)\cong\mathbb{C}.$$

If I'm not mistaken, it's fairly straightforward to show that $$\text{dim }H^1(C, \Omega^1_C)=1$$ via the Riemann-Roch theorem. I'm getting a bit stuck with showing the canonical isomorphism however.

My attempt

Since $$C$$ is projective, we can define a closed embedding $$C\hookrightarrow\mathbb{P}^n$$. Let $$\mathcal{I}_C$$ be the corresponding ideal sheaf and consider the conormal short exact sequence $$0\to \mathcal{I}_C/\mathcal{I}_C^2\to \Omega^1_{\mathbb{P}^n}\otimes \mathcal{O}_C\to \Omega^1_C\to 0.$$ This induces a long exact sequence in cohomology $$\cdots\to H^1(C,\mathcal{I}_C/\mathcal{I}_C^2)\to H^1(C, \Omega^1_{\mathbb{P}^n}\otimes \mathcal{O}_C)\to H^1(C,\Omega^1_C)\to H^2(C,\mathcal{I}_C/\mathcal{I}_C^2)\to \cdots$$ But $$H^2(C,\mathcal{I}_C/\mathcal{I}_C^2)$$ vanishes (along with all other higher terms) for dimension reasons. Furthermore, I think I can show that $$H^1(C, \Omega^1_{\mathbb{P}^n}\otimes \mathcal{O}_C)\cong \mathbb{C}$$ via an Euler sequence argument.

If I could also show that $$H^1(C,\mathcal{I}_C/\mathcal{I}_C^2)=0$$ then I would be done, but it's this part I'm stuck on. I thought perhaps of using Serre duality but I didn't have much success.

Any help is much appreciated!

This is a direct consequence of Serre duality: $$H^1(C, \Omega^1_C) \cong H^0(C, \mathcal{O}_C)^{\vee} \cong \mathbb{C}$$ where the last isomorphism comes from the projectivity of $$C$$. The duality pairing also gives you that the isomorphism is canonical.
• It comes from the canonical morphism $\mathbb{C}\rightarrow \mathrm{Hom}(\mathcal{O}_C,\mathcal{O}_C)$. Commented Jun 8, 2021 at 22:46