Topologically contractible algebraic varieties

Question

Are all topologically trivial (contractible) complex algebraic varieties necessarily affine? Are there examples of those not birationally equivalent to an affine space?

The examples that come to my mind are similar to a singular $\mathbb P^1$ without a point given by equation $x^2 = y^3$. This particular curve is clearly birationally equivalent to affine line.

Perhaps the "affine" part follows from a comparison between Zariski cohomology and complex cohomology?

Same comment as on that one, you mean affine SPACE in your question, not affine line. $\mathbb{A}^2$ is definitely contractible. — Charles Siegel, Dec 02 '09 at 19:34

score 26 · Accepted Answer · edited Jul 16 '22 at 03:33

26

No. Counterexamples were first constructed by Winkelmann, as quotients of $\mathbb A^5$ by algebraic actions of $\mathbb G_{\text{a}}$. I learned this from Hanspeter Kraft's very nice article available here:

Challenging problems on affine $n$-space.

Recently Aravind Asok and Brent Doran have been studying these kinds of examples in the setting of $\mathbb A^1$-homotopy theory, on the arxiv as On unipotent quotients and some A^1-contractible smooth schemes.

edited Jul 16 '22 at 03:33

LSpice

11,423

answered Dec 02 '09 at 19:55

David Treumann

4,849

This is a very interesting example -- I wonder if it's also not birational to affine line? – Ilya Nikokoshev Dec 02 '09 at 20:22
The first link's paper is really nice! – Csar Lozano Huerta Dec 03 '09 at 06:19
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The link to H. Kraft's paper is broken – Qfwfq Feb 25 '16 at 09:33
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@Qfwfq, I fixed the link to Kraft's paper (I presume that I have the right one, and there is also a citation to Winkelmann's paper there). – Peter McNamara Feb 25 '16 at 22:26

score 11 · Answer 2 · edited Jul 16 '22 at 03:34

About the rationality of contractible varieties: Yes for curves and surfaces and is an open question for higher dimensions.

Any such contractible variety $X$ has $\chi_\text{top}(X)=1$, obviously.

If $X$ is a curve then it must have only cusps as singularities, if any, by a simple $\chi_\text{top}$ calculation. Now let $Y$ be a projective model of $X$ such that it is smooth at the points in $Y-X$. Topologically, $Y$ is a real surface without boundary such that a few punctures make it contractible. The only real surface with this property is $S^2$, obviously. Hence $Y$ better be rational and so is $X$.

If $X$ is an algebraic surface then it was a conjecture of Van de Ven that such a surface must be rational (actually his conjecture is for any homologically trivial $X$). This was proved by Gurjar & Shastri in:

On the rationality of complex homology 2-cells
Here is the the part II of the above paper (MathSciNet review number MR0984747)

Topologically contractible algebraic varieties

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