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A stupid question: which statements in section 5 of BBD will fail if we replace $\overline{\mathbb{Q}_l}$-sheaves by just $\mathbb{Q}_l$-ones? I am especially interested in Proposition 5.1.15.

BBD = Beilinson A., Bernstein J., Deligne P., Faisceaux pervers, Asterisque 100, 1982, 5-171.

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  • $\begingroup$ Given the buzzwords in this question, I am not in its intended audience. However, I think it would be useful to say what BBD is. $\endgroup$ Jul 10, 2010 at 7:06

2 Answers 2

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I think that all the statements are true, except for 5.3.9 (ii). Remark 5.3.10 says that all the statements in 5 up to and including 5.3.8 are true for $\mathbb{Q}_\ell$-coefficients with the same proof, and that 5.3.9 (i) is still true but with a slightly more complicated proof. I am pretty sure that the proof of corollary 5.3.10 works too for $\mathbb{Q}_\ell$-coefficients. I cannot see any reason why the proofs in 5.4 would not also work, but I have read them quickly. Anyway, proposition 5.1.15 is certainly fine.

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Most of the statements go through. This is discussed in some detail at the start of Chapter 4. They also give a list of things that one has to be careful of when using other coefficients.

In particular, I think Proposition 5.1.15 stays true with coefficients in $\mathbb{Q}_{\ell}$. This should follow because all functors commute with the extension of scalars functor from $\mathbb{Q}_{\ell}$ to $\overline{\mathbb{Q}}_{\ell}$.

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  • $\begingroup$ It seems that the restrictions mentioned are not actual for $\mathbb{Q}_l$-coefficients? $\endgroup$ Jul 10, 2010 at 16:51
  • $\begingroup$ Yes, exactly. I also think one can see this directly (as I have tried to hint at above). $\endgroup$ Jul 12, 2010 at 8:06

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