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Let $DM_{\text{gm}}$ be the category of Voevodsky´s geometric motives. Let $p,q\in \mathbb{Z}$ be integers with $p<0$.

Is it true that $$\text{Hom}_{DM_{\text{gm}}}(M_{\text{gm}}(X),\mathbb{Z}(p)[q])=0,$$ where $M(X)$ is the motive of a smooth scheme $X$ over a field $k$ and $\mathbb{Z}(p)$ is the Tate motive?

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    $\begingroup$ By definition, negative Tate twists do not exist in the category of effective motives, so that this question does not even make sense. $\endgroup$ Commented Sep 4, 2018 at 14:31
  • $\begingroup$ Dear @Denis-CharlesCisinski, thank you for your answer. I was trying to understand the last part of the first point of Lemma 7.1.1 in uni-due.de/~bm0032/publ/Azumaya24.pdf $\endgroup$
    – user438991
    Commented Sep 4, 2018 at 15:25
  • $\begingroup$ If you drop the effectiveness, this vanishing makes sense, does hold, and is used in the proof of the second assertion of Lemma 7.1.1 of loc. cit. indeed. $\endgroup$ Commented Sep 4, 2018 at 16:43
  • $\begingroup$ @Denis-CharlesCisinski. I see now. So, the vanishing, droping the effectiveness, follows by construction? Or do they invoke some well known result? $\endgroup$
    – user438991
    Commented Sep 4, 2018 at 17:19
  • $\begingroup$ This vanishing property does not hold by construction but is well known indeed. $\endgroup$ Commented Sep 4, 2018 at 22:00

2 Answers 2

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Here's an easy way to see this (which is more or less an elaboration of Mikhail's answer):

Suppose $i > 0$ and $n\in \mathbb Z$. We have the Thom isomorphism $$ H^n(X, \mathbb Z(-i)) \cong H^{n+2i}_X(\mathbb A^i_X, \mathbb Z(0)). $$ Weight $0$ motivic cohomology is just Zariski cohomology, so $$ H^n(X,\mathbb Z(0)) = \begin{cases} \mathbb Z^{\pi_0(X)} & \text{if }n=0,\\ 0 & \text{otherwise.}\end{cases} $$ Now the long exact sequence for cohomology with support shows that $H^*_X(\mathbb A^i_X,\mathbb Z(0))=0$.

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I will sketch a proof.

It suffices to prove that there are only zero morphisms from $M_{\text{gm}}(X)(1)$ into $\mathbb{Z}[q]$ for any smooth $X$ and $q\in \mathbb{Z}$. The latter statement easily follows from the fact that the $\mathbb{Z}$ is a birational sheaf (with transfers); see Lemma 2.3.2(b) of "Birational Motives, II: Triangulated Birational Motives" by Bruno Kahn and Ramdorai Sujatha.

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