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K-theory and Noncommutative Geometry.pdf

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Duality for topological abelian group stacks and T -duality 259Proof. Indeed, for .V ! U/2 S=U we have.f G/ jU .V ! U/D G.f .V //D G jf.U/ .f .V / ! f.U//D . U f G jf.U/ /.V ! U/:In order to see the second identity note that we can writeF jU Š colim .A!FjU /2Sh.S=U /=F jUA:Since U f is a left-adjoint it commutes with colimits. Furthermore the restrictionfunctors .:::/ jU and .:::/ f.U/ are exact by Lemma 3.8 and therefore also commutewith colimits. WritingF Š colim .A!F/2Sh.S/=F Awe getF jU Š .colim .A!F/2Sh.S/=F A/ jU Š colim .A!F/2Sh.S/=F A jUŠ colim .A!F/2Sh.S/=F A U ! A (by Lemma 3.6).Using that f preserves products in the isomorphism marked by .Š/ we calculateU f F jU Š U f colim .A!F/2Sh.S/=F A U ! UŠ colim .A!F/2Sh.S/=F U f A U ! UŠ colim .A!F/2Sh.S/=F f.A U/! f.U/ (by Lemma 3.17).Š/Š colim .A!F/2Sh.S/=F f .A/ f.U/! f.U/Š colim .A!F/2Sh.S/=F f .A/ jf.U/Š .colim .A!F/2Sh.S/=F f .A// jf.U/Š .f F/ jf.U/ : (by Lemma 3.16)Lemma 3.20. Assume that S; S 0 have finite products which are preserved by f W S ! S 0 .For F 2 ShS and G 2 ShS 0 we have a natural isomorphismProof. For U 2 S we calculateHom Sh S .F; f G/ Š f Hom Sh S 0.f F;G/:Hom Sh S .F; f G/.U / D Hom Sh S=U .F jU ;.f G/ jU /Š Hom Sh S=U .F jU ; U f .G jf.U/ // (by Lemma 3.19)Š Hom Sh S 0 =f .U /. U f .F jU /; G jf.U/ /Š Hom Sh S 0 =f .U /..f F/ jf.U/ ;G jf.U/ / (by Lemma 3.19)D Hom Sh S 0.f F; G/.f .U //D f Hom Sh S 0.f F; G/.U /

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