De rham cohomology over complex manifolds

complex-geometryde-rham-cohomologysheaf-cohomology

I am studying sheaf cohomology of complex manifolds and, while reading some proof about Dolbeault cohomology, I realized that there is a $\bar{\partial}$-Poincaré Lemma which gives us the local exactness of the complex we use when talking about Dolbeault cohomology and so allows us to use sheaf cohomology.

It's not entirely clear to me, however, if we also have a $\partial$-Poincaré Lemma (I think we have it though, since Cauchy formula can be generalized to higher dimension using polydisks), and assuming we have it, can we use it to create a "complex de Rham cohomology" which is based entirely on holomorphic forms and doesn't use smooth ones? And if so, what is its relation with the standard de Rham cohomology of the underlying real manifold?

Best Answer

Here is a version of the $\bar \partial$-Poincaré lemma as formulated in [1]

Proposition 2.31 Let $\alpha$ be a $C^1$-form of type $(p,q)$ with $q > 0$. If $\bar \partial \alpha = 0$, then there locally exists on $X$ a $C^1$-form $\beta$ of type $(p,q-1)$ such that $\alpha = \bar \partial \beta$.

Now to your question:

It's not entirely clear to me, however, if we also have a ∂-Poincarè Lemma, and assuming we have it, can we use it to create a "complex de rham cohomology" which is based entirely on holomorphic forms and doesn't use smooth ones?

This is not immediately possible, because if you formulate a similar $\partial$-Poincaré lemma, and apply it to a holomorphic form $\alpha$, you do not know if $\beta$ is holomorphic as well. However, it is still true that one can build a holomorphic de Rham complex, using the sheaves $\Omega^k$ of holomorphic differentials. It looks like one would expect: $$0 \to \mathbb C_X \to \mathcal O_X \xrightarrow{\partial} \Omega^1_X \xrightarrow{\partial} \Omega^2_X \to \dotsc \xrightarrow{\partial} \Omega^n_X \to 0$$ To show that this is an exact complex of sheaves, one can for each $k$ look at the resolution $$0 \to \Omega^k \xrightarrow{\bar \partial} \mathcal A^{k,0} \xrightarrow{\bar \partial} \mathcal A^{k,1} \dotsc$$ and those are exact by the $\bar \partial$-Poincaré lemma. Here $\mathcal A^{p,q}$ is the sheaf of $C^\infty$-forms of type $(p,q)$. Together they form a double complex $$(A^{\bullet, \bullet}, \partial, \bar \partial),$$ whose total complex is the de Rham complex $(\mathcal A^\bullet, d)$ with complex coefficients, as $\mathcal A^k = \bigoplus_{p+q=k} \mathcal A^{p,q}$. All of this shows that the holomorphic de Rham complex $(\Omega^\bullet, \partial)$ is quasi-isomorphic to the complexified de Rham complex. Hence it computes the de Rham cohomology $$H^k_{dRh}(X) \otimes \mathbb C = H^k(X, \mathbb C_X),$$ where on the right hand side I mean sheaf cohomology.


[1] Voisin, Hodge Theory and Complex Algebraic Geometry, I