# naive de Rham cohomology fails for singular varieties

Let $X$ be a variety over a field $k$ of characteristic zero. If $X$ is smooth, algebraic de Rham cohomology defined as $$H^n_{dR}(X / k)=\mathbb{H}^n(X, \Omega^\bullet_{X/k})\qquad (\star)$$ is a good cohomological theory, for instance, is isomorphic tensor $\mathbb{C}$ to the singular cohomology of $X(\mathbb{C})$ once one chooses and embedding $k \hookrightarrow \mathbb{C}$.

It is "well known" that this fails for singular $X$ unless one considers more sophisticated definitions of de Rham cohomology, but I had difficulties finding a counterexample of, say, a singular variety $X$ such that the dimension of $(\star)$ is different from the dimension of singular cohomology. After a while, I found a paper by Arapura-Kang where the example of the plane curve $$x^5+y^5+x^2y^2=0$$ is presented. The proof is not so easy. Does anybody know a simpler example?

• We found ourselves in the same position as you a few years ago, and played around until we found this. But I agree that there ought to be something easier. Dec 4, 2016 at 14:14
• Great, a comment from the source himself! Thanks for writing that paper, I learned a lot from it
– dr91
Dec 4, 2016 at 14:20
• See also this question Sep 14, 2017 at 0:54

## 1 Answer

The nonexactness of the naïve analytic de Rham complex for complex analytic spaces is discussed in the following papers.

H.J. Reiffen, Das Lemma von Poincaré fȕr holomorphs Differential-formen auf komplexen Räumen, Math Z. 101 (1967).

M. Sebastiani, Preuve d'une Conjecture de Brieskorn, Manuscripta math. 2 (1970).

K. Saito, Quasihomogene isolierte Singularitäten von Hyperflächen, Invent. math. 14 (1971).

There are many isolated hypersurface singularities which are not quasihomogeneous (in the sense that there are no local analytic coordinates in which the hypersurface is locally defined by a quasihomogeneous polynomial) and for any such one can globalize it to a projective complex hypersurface with one singular point, and by the above in this case the Euler characteristic of the singular cohomology differs from the Euler characteristic of the naïve de Rham cohomology.