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K-theory and Noncommutative Geometry.pdf

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Axioms for the norm residue isomorphism 433Corollary 3.3. The structural map H 1; 1 .M / ! H 1; 1 .k/ D k is injective.Proof. By (0.4) and Lemma 3.2, it suffices to show that Hom.Z;D.bC 1/Œ2b C 1/ D 0.By (0.5) this results from the vanishing of both Hom.Z;M.b C 1/Œ2b C 1/ andHom.Z; X ˝ L bCdC1 / which follows from Lemma 3.1.Lemma 3.4. H 0;1 .X/ D H 2;1 .X/ D 0 and H 1;1 .XI Z/ Š H 1;1 .Spec kI Z/ Š k .Proof. The spectral sequence E p;q1D H q .X pC1 I Z.1// ) H pCq;1 .XI Z/ degenerates,all rows vanishing except for q D 1 and q D 2, because Z.1/ Š O Œ 1; see[2, 4.2]. We compare this with the spectral sequence converging to H pCq .XI G m /;Hzar q .Y; O / ! H q ét . ; G m/ is an isomorphism for q D 0; 1 (and an injection forq D 2). Hence for q 2 we have H q;1 .X/ D H q;1ét.X/ D H q;1ét.k/ D H q 1ét.k; G m /.Remark. The proof of 3.4 also shows that H 3;1 .X/ injects into H 2 ét .k; G m/ D Br.k/.Lemma 3.5. H 2dC1;dC1 .DI Z/ is the kernel of H 1; 1 .M /y! H 1; 1 .k/ D k .Proof. From (0.5) we get an exact sequence with coefficients Z:H 2d;dC1 .X˝L d / ! H 2dC1;dC1 .D/ ! H 2dC1;dC1 .M / Dy ! H 2dC1;dC1 .X˝L d /:The first group is H 0;1 .X/, which is zero by 3.4, so it suffices to show that the map Dyidentifies with the structural map y. This follows from Axiom 0.3 (b), because for anyu in H 1; 1 .M / D Hom.Z; M.1/Œ1/, X tensored with the compositeX ˝ L d Dy ! M ˝ L d u ! Z.1/Œ1 ˝ L d D Z.d C 1/Œ2d C 1is the X-dual of X. 1/Œ 1u! M y ! X.Corollary 3.6. H 2dC1;dC1 .D/ D H 2dC1;dC1 .DI Z/ ˝ Z .`/ D 0.4 Between motivic and étale cohomologyDefinition 4.1 ([10, p. 90]). Let L.n/ denote the truncation nC1 Z ét .n/ of the complexin DM eff representing étale motivic cohomology; i. e., H p . ; L.n// Š H p;nét.`/. /for p n C 1. Let K.n/ denote the mapping cone of the canonical map Z .`/ .n/ !L.n/, and consider the triangle Z .`/ .n/ ! L.n/ ! K.n/ ! Z .`/ .n/Œ1.Lemma 4.2. The map H nC1 .X; K.n//y! H nC1 .M; K.n// is an injection.Proof. By triangle (0.4) we have an exact sequence:H n .D ˝ L b ; K.n// ! H nC1 .X; K.n//y! H nC1 .M; K.n//:

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