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Recall that a variety $X$ over a finite field $k$ is said to be pure if the eigenvalues of the Frobenius on $i^{\mathrm{th}}$ etale cohomology of $\overline{X}:=X\otimes_k \overline{k}$ have absolutely value $q^{\frac{i}{2}}$.

Part III of the Weil conjectures (proved by Deligne) states that every smooth projective variety is pure.

There are, however, interesting examples of pure varieties which are not smooth projective. For instance, Springer fibres are singular projective and pure. Similarly, quiver varieties are smooth not projective but pure.

Question: What are some examples of varieties which are neither smooth nor projective, but that are nonetheless pure?

It would be helpful if a brief explanation is given regarding how one proves purity in these examples (and/or a reference).

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  • $\begingroup$ Yeah sure...a bit more interesting examples would be appreciated :) $\endgroup$
    – Dr. Evil
    Commented Jan 27 at 8:38
  • $\begingroup$ If there exists a radiciel resolution of singularities (e.g., normalization of a cusp) then the etale cohomology is pure. $\endgroup$ Commented Jan 27 at 12:31
  • $\begingroup$ Would it maybe be more natural to consider cohomology with compact supports for non-proper varieties? This is what appears in the Grothendieck-Lefschetz trace formula, for example. $\endgroup$ Commented Jan 29 at 10:44
  • $\begingroup$ Good point Daniel. Sure we can do that. $\endgroup$
    – Dr. Evil
    Commented Jan 31 at 20:12

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I don't know about interesting, but here are a few easy examples. Start with a cone $V\subset \mathbb{A}^n_k$. This is easily seen to be $\mathbb{A}^1$-homotopy equivalent to a point, so $H^i(\overline{V}, \mathbb{Q}_\ell)=0$. So it's pure, but for boring reasons. You can make make a slightly more interesting example by taking a bundle of cones over a smooth projective variety $X$ (e.g. $(V\times L)/\mathbb{G}_m$, where $L$ is a $\mathbb{G}_m$-bundle over $X$).

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