# De Rham cohomology

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of an algebraic variety

A cohomology theory of algebraic varieties based on differential forms. To every algebraic variety $X$ over a field $k$ is associated a complex of regular differential forms (see Differential form on an algebraic variety); its cohomology groups $H_{\text{dR}}^p (X/k)$ are called the de Rham cohomology groups of $X$. If $X$ is a smooth complete variety and if $\text{char } k=0$, then de Rham cohomology is a special case of Weil cohomology (see , ). If $X$ is a smooth affine variety and if $k = \mathbf{C}$, then the following analogue of the de Rham theorem is valid:

$$H_{\text{dR}}^p (X/k) \cong H^p (X^\text{an}, \mathbf{C}), \qquad p \ge 0,$$

where $X^\text{an}$ is the complex-analytic manifold corresponding to the algebraic variety $X$ (see ). For example, if $X$ is the complement of an algebraic hypersurface in $P^n(\mathbf{C})$, then the cohomology group $H^p(X, \mathbf{C})$ can be calculated using rational differential forms on $P^n(\mathbf{C})$ with poles on this hypersurface.

For any morphism $f:X\to S$ it is possible to define the relative de Rham complex $\sum_{p\ge 0} \Gamma(\Omega_{X/S}^p)$ (see Derivations, module of), which results in the relative de Rham cohomology groups $H_{\text{dR}}^p (X/S)$. If $X=\text{Spec } A$ and $S=\text{Spec } B$ are affine, the relative de Rham complex coincides with $\Lambda \Omega_{A/B}^1$. The cohomology groups $\mathcal{H}_{\text{dR}}^p (X/S)$ of the sheaf complex $\sum_{p\ge 0} f \ast \Omega_{X/S}^p$ on $S$ are called the relative de Rham cohomology sheaves. These sheaves are coherent on $S$ if $f$ is a proper morphism.