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K-theory and Noncommutative Geometry.pdf

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386 P. Bressler, A. Gorokhovsky, R. Nest, and B. TsyganLemma 7.13. There exists a unique ! 2 .XI . 2 X ˝ J X=O X / cl / such thatr can F p D p ! ˝ Id p;where Id p denotes the .p C 1/ .p C 1/ identity matrix.Proof. Using the definition of F 0 and formula (7.2) we obtain: .d 1 / r can F 0.d 0 / r can F 0 Dr can F.;r/ 2 2 .N 1 U/. Hence .d 1 / r can F 0 .d 0 / r can F 0 D 0,and there exists a unique ! 2 .XI X 2 ˝ .J X=O X // such that 0 ! D rcan F 0 .Since .r can / 2 D 0 it follows that r can ! D 0. For any p we have: .r can F p / ii Dpr i rcan F 0 D p !, and the assertion of the lemma follows.Lemma 7.14. The class of ! in H 2 ..XI X ˝ J X=O X /; r can / does not depend onthe choices made in its construction.Proof. The construction of ! is dependent on the choice of F 0 , 2 Isom 0 .A 01 ˝J N1 UJ.A 01 //, and r 2 C .A 01 / satisfying the equation (7.6). Assume that wemake different choices: 0 D .@/ L , r 0 D .@˛/ L rand .F 0 / 0 satisfying @.F L 0 / 0 DF. 0 ; r 0 /. Here, 2 xC 0 .UI J 0 / and ˛ 2 xC 0 .UI 1 /. We have: F. 0 ; r 0 / DF.;r/ @˛ L C @r L can . It follows that @..F L 0 / 0 F 0 r can C ˛/ D 0. Therefore.F 0 / 0 F 0 r can C ˛ D 0ˇ for some ˇ 2 .XI 1 X ˝ J X/. Hence if ! 0 isconstructed using 0 , r 0 , .F 0 / 0 then ! 0 ! Dr can Nˇ where Nˇ is the image of ˇ underthe natural projection .XI X 1 ˝ J X/ ! .XI X 1 ˝ .J X=O X //.Let W V ! U be a refinement of the cover U, and .V; A / the correspondingdescent datum. Choice of , r, F 0 on U induces the corresponding choice .N/ ,.N/ r, .N/ .F 0 / on V. Let ! denotes the form constructed as in Lemma 7.13using .N/ , .N/ r, .N/ F 0 . Then,! D .N/ !:The following result now follows easily and we leave the details to the reader.Proposition 7.15. The class of ! in H 2 ..XI X ˝ J X=O X /; r can / coincides withthe image ŒS of the class of the gerbe.7.5 Construction of the quasiisomorphism. For W Œn ! letH WD .N .n/ UI N .n/ U ˝ .0n/ C .Mat.A/ .0/ ˝ J N.0/ U/ loc Œ1/;considered as a graded Lie algebra. For W Œm ! Œn, D ı there is a morphismof graded Lie algebras W H ! H .Forn D 0;1;:::let H n WD Q H . TheŒn! assignment 3 Œn 7! H n , 7! defines a cosimplicial graded Lie algebra H.For each W Œn ! the map WD Id ˝ .0n/ . .0/ /W H ! G DR .J.A//

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