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K-theory and Noncommutative Geometry.pdf

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258 U. Bunke, T. Schick, M. Spitzweck, and A. Thom3.3.14 Let f W S ! S 0 be a morphism of sites. It induces an adjoint pairf W ShS , ShS 0 W f :Lemma 3.16. For F 2 ShS the functor f has the explicit descriptionProof. In fact, for G 2 ShS 0 we havef .F / WD colim .U !F/2Sh.S=F / f.U/:Hom Sh S 0.colim .U !F/2Sh.S/=F f.U/;G/Š lim .U !F/2Sh.S/=F Hom Sh S 0.f .U /;G/Š lim .U !F/2Sh.S/=F G.f.U//Š lim .U !F/2Sh.S/=F f G.U /Š lim .U !F/2Sh.S/=F Hom Sh S 0.U ;f G/Š Hom Sh S .colim .U !F/2Sh.S/=F U ;f G/Š Hom Sh S .F; f G/:Lemma 3.17. For A 2 S we havef .A/ D f .A/:Proof. Since .id A W A ! A/ 2 S=A is final we havef .A/ Š colim .U !A/2Sh.S/=A f.U/Š colim .U !A/2S=A f.U/Š f .A/:Lemma 3.18. If f W S ! S 0 is fully faithful, then we have for A 2 S that f f A Š A.Proof. We calculate for U 2 S thatf f A.U / Š f f .A/.U / (by Lemma 3.17)Š f .A/.f .U //Š Hom S 0.f .U /; f .A//Š Hom S .U; A/Š A.U /3.3.15 For U 2 S we have an induced morphism of sites U f W S=U ! S 0 =f .U /which maps .A ! U/to .f .A/ ! f.U//.If f is fully faithful, then U f is fully faithful for all U 2 S.Lemma 3.19. For G 2 ShS 0 we have in S=U the identityU f .G jf.U/ / Š .f G/ jU :If we know in addition that S, S 0 have finite products which are preserved by f W S ! S 0 ,then for F 2 S we have in S 0 =f .U / the identityU f F jU Š .f F/ jf.U/ :

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