# excision in algebraic de Rham cohomology

Hi friends,

I'm reading the notes by Kedlaya "p-adic cohomology from theory to practice", available here

In Remark 1.3.7 on page 6 he says that if $X$ is a smooth algebraic variety over a field $k \subset \mathbb{C}$ and $Z$ is a smooth subvariety of pure codimension $d$, then there exists a excision sequence in algebraic de Rham cohomology

$$\cdots \to H^{i−2d}_{dR}(Z) \to H^i_{dR}(X) \to H^i_{dR}(X-Z)\to H^{i−2d+1}_{dR}(Z) \to \cdots$$

Question 1: I was completely unable to find a reference for this. Can anyone provide a reference?

Question 2: I guess one also has an excision sequence of the same form in the singular cohomology of $X(\mathbb{C})$ seen as a complex variety. Are both sequence compatible by the isomorphism period?

Question 3: Is it possible to get rid of the shift by codimension by considering the compact support cohomology of $X-Z$?

Any help will be appreciated!

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You should find the answers to your questions in Hartshorne's article "On the de Rham cohomology of algebraic varieties", esp. p. 40, p. 47, p. 50. –  Damian Rössler Feb 15 '13 at 18:10