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Such an example of course could not be projective and would not itself lift to Z_p. The context is that one can compute p-adic cohomology of a variety X over a finite field F_p via the cohomology of an embedding of X into a smooth Z_p scheme.

This is similar in spirit to my questions here and here (but a different question than the second link).

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3 Answers 3

up vote 4 down vote accepted

EDIT 7/15/14 I was just looking back at this old answer, and I don't think I ever answered the stated question. I can't delete an accepted answer, but I'll point at that, as far as I can tell, the Vakil reference I give also only address the question of deforming $X$ over $\mathbb{Z}_p$, not of embedding it in some larger flat family over $\mathbb{Z}_p$.

EDIT Oops! David Brown points out below that I misread the question. I was answering the question of finding a smooth scheme which does not deform in a smooth family over Z_p.

Well, to make up for that, I'll point to some references which definitely contain answers. Look at section 2.3 of Ravi Vakil's paper on Murphy's law for deformation spaces http://front.math.ucdavis.edu/0411.5469 for some history, and several good references. Moreover, Ravi describes how to build an explicit cover of P^2 in characteristic p which does not deform to characteristic 0. Basically, the idea is to take a collection of lines in P^2 which doesn't deform to characteristic 0 and take a branched cover over those lines. For example, you could take that p^2+p+1 lines that have coefficients in F_p.

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David, I think you misread the question (or maybe I am confused). If X is projetive, then it embeds into P^n over Z_p, which is a smooth Z_p scheme. Ravi's example (I believe) is of a variety this doesn't lift to a variety over Z_p. –  David Zureick-Brown Oct 15 '09 at 5:10
    
David edited his answer so that my comment no longer makes sense, but I can't edit my comment to reflect that so I'll leave this comment instead. –  David Zureick-Brown Oct 16 '09 at 5:52
    
I had a bounty on this question and had to accept your answer! –  David Zureick-Brown Jul 15 at 18:20
    
Hmm, can you switch your answer to ulrich's now, and I'll delete mine? (Sorry for not thinking clearly back in '09.) –  David Speyer Jul 15 at 21:23
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A theorem of Wlodarczyk in "Embedding varieties in toric varieties" says that any smooth variety such that any two points are contained in an affine open set can be embedded in a smooth toric variety. Toric varieties can be lifted to Z_p so any variety over F_p with the above property can be embedded in a smooth scheme over Z_p.

Unfortunately, not all smooth varieties have this property; the example in Hartshorne of a smooth proper 3-fold which is not projective appears to be one where this fails (though for suitable choices these could lift to Z_p).

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I know that Kiran Kedlaya knows the answer to this question. You might e-mail him.

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Also, if you get an informative response, please post it here. –  S. Carnahan Oct 13 '09 at 16:01
    
Kiran didn't know off the top of his head, but said that he believes should exist. –  David Zureick-Brown Oct 14 '09 at 14:58
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