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Noncommutative motives, numerical equivalence, and semi-simplicity
- American Journal of Mathematics
- Johns Hopkins University Press
- Volume 136, Number 1, February 2014
- pp. 59-75
- 10.1353/ajm.2014.0004
- Article
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Making use of Hochschild homology, we introduce the correct category ${\rm NNum}(k)_F$ of
noncommutative {\it numerical} motives (over a base ring $k$ and with coefficients in a field $F$).
We prove that ${\rm NNum}(k)_F$ is abelian semi-simple and that Grothendieck's category
${\rm Num}(k)_{\Bbb{Q}}$ of numerical motives embeds into ${\rm NNum}(k)_{\Bbb{Q}}$ after being
factored out by the action of the Tate object. As an application we obtain an
alternative proof of Jannsen's celebrate semi-simplicity result, which uses the
noncommutative world instead of a classical Weil cohomology.