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K-theory and Noncommutative Geometry.pdf

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Duality for topological abelian group stacks and T -duality 3175.1.7 Assume now that F is dualizable and admissible (at this point we only needExt 1 Sh Ab S .F; T/ Š 0). Then we haveD.D.ch.F /// Š D.B.D.F /// (by Lemma 5.6)Š ch.D.D.F // (by Lemma 5.5)Š ch.F /:Similarly, if F is dualizable and D.F / admissible (again we only need the weakercondition that Ext 1 Sh Ab S .D.F /; T/ Š 0), then we have, again by Lemmas 5.5 and 5.6,D.D.BF//Š D.ch.D.F /// Š BD.D.F // Š BF:5.1.8 Let us now formalize this observation. Let P 2 PIC.S/ be a Picard stack.Definition 5.7. We call P dualizable if the natural evaluation morphism P ! D.D.P //is an isomorphism.The discussion of 5.1.7 can now be formulated as follows.Corollary 5.8. 1. If F is dualizable and admissible, then ch.F / is dualizable.2. If F is dualizable and D.F / is admissible, then BF is dualizable.The goal of the present subsection is to extend this kind of result to more generalPicard stacks.5.1.9 Let P 2 PIC.S/.Lemma 5.9. We haveH 1 .D.P // Š D.H 0 .P //and an exact sequence0 ! Ext 1 Sh Ab S .H 0 .P /; T/ ! H 0 .D.P // ! D.H 1 .P // ! Ext 2 Sh Ab S .H 0 .P /; T/:(45)Proof. By Lemma 2.13 we can choose K 2 C.Sh Ab S/ such that P Š ch.K/. WenowgetH 1 .D.P // Š R 1 Hom ShAb S .K; TŒ1/ (by Lemma 2.18)Š R 0 Hom ShAb S .K; T/Š Hom ShAb S .H 0 .K/; T/Š D.H 0 .P //:Again by Lemma 2.18 we haveH 0 .D.P // Š R 0 Hom ShAb S .K; TŒ1/ Š R1 Hom ShAb S .K; T/:

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