A Journal of the K-Theory Foundation(Ktheoryfoundation.Org)
Total Page:16
File Type:pdf, Size:1020Kb
ANNALSOF K-THEORY Paul Balmer Spencer Bloch no. 1 vol. 1 2016 Alain Connes Guillermo Cortiñas Eric Friedlander Max Karoubi Gennadi Kasparov Alexander Merkurjev Amnon Neeman Jonathan Rosenberg Marco Schlichting Andrei Suslin Vladimir Voevodsky Charles Weibel Guoliang Yu msp A JOURNAL OF THE K-THEORY FOUNDATION ANNALS OF K-THEORY msp.org/akt EDITORIAL BOARD Paul Balmer University of California, Los Angeles, USA [email protected] Spencer Bloch University of Chicago, USA [email protected] Alain Connes Collège de France; Institut des Hautes Études Scientifiques; Ohio State University [email protected] Guillermo Cortiñas Universidad de Buenos Aires and CONICET, Argentina [email protected] Eric Friedlander University of Southern California, USA [email protected] Max Karoubi Institut de Mathématiques de Jussieu – Paris Rive Gauche, France [email protected] Gennadi Kasparov Vanderbilt University, USA [email protected] Alexander Merkurjev University of California, Los Angeles, USA [email protected] Amnon Neeman amnon.Australian National University [email protected] Jonathan Rosenberg (Managing Editor) University of Maryland, USA [email protected] Marco Schlichting University of Warwick, UK [email protected] Andrei Suslin Northwestern University, USA [email protected] Vladimir Voevodsky Institute for Advanced Studies, USA [email protected] Charles Weibel (Managing Editor) Rutgers University, USA [email protected] Guoliang Yu Texas A&M University, USA [email protected] PRODUCTION Silvio Levy (Scientific Editor) [email protected] Annals of K-Theory is a journal of the K-Theory Foundation(ktheoryfoundation.org). The K-Theory Foundation acknowledges the precious support of Foundation Compositio Mathematica, whose help has been instrumental in the launch of the Annals of K-Theory. See inside back cover or msp.org/ant for submission instructions. The subscription price for 2016 is US $400/year for the electronic version, and $450/year (C$25, if shipping outside the US) for print and electronic. Subscriptions, requests for back issues and changes of subscribers address should be sent to MSP. Annals of K-Theory (ISSN 2379-1681 electronic, 2379-1683 printed) at Mathematical Sciences Publishers, 798 Evans Hall #3840, c/o University of California, Berkeley, CA 94720-3840 is published continuously online. Peri- odical rate postage paid at Berkeley, CA 94704, and additional mailing offices. AKT peer review and production are managed by EditFlow® from MSP. PUBLISHED BY mathematical sciences publishers nonprofit scientific publishing http://msp.org/ © 2016 Mathematical Sciences Publishers ANNALS OF K-THEORY Vol. 1, No. 1, 2016 msp dx.doi.org/10.2140/akt.2016.1.1 Statement of purpose The Annals of K-Theory (AKT) has been established to serve as the premier journal in K-theory and associated areas of mathematics. These include areas of algebraic geometry, homological algebra, category theory, geometry, functional analysis, and algebraic topology, encompassing such topics as cyclic homology, motivic homotopy theory, KK-theory, index theory, and more. The journal wel- comes strong submissions in all areas in which K-theory concepts or methodology play a role. AKT will follow a rigorous editorial process, with an Editorial Board of experts, and an elected managing committee. Papers recommended by members of the board are forwarded to the managing committee, which reviews them again on the basis of the recommendation of the handling editor, the external referee report(s), and the managing committee’s own impressions. Then discussion is opened to the entire Editorial Board, which makes a collective decision. In this way we hope to adhere to the highest scientific and expository standards. The content of AKT, and the editorial process, is managed by the K-Theory Foundation, Inc. (KTF), a nonprofit organization run by mathematicians. The in- come produced by the journal will be used by the KTF to fund activities benefiting the K-theory community, such as conferences, summer schools, and prizes for deserving young mathematicians. Another nonprofit, Mathematical Sciences Publishers, will handle the copy- editing, publication, and distribution. The KTF is grateful to yet a third nonprofit, the Foundation Compositio Mathematica, for its support in helping to get the jour- nal up and running. All three nonprofit organizations are run by mathematicians for mathematicians. Jonathan M. Rosenberg Charles A. Weibel ANNALS OF K-THEORY Vol. 1, No. 1, 2016 msp dx.doi.org/10.2140/akt.2016.1.3 On the Deligne–Beilinson cohomology sheaves Luca Barbieri-Viale qC1 We prove that the Deligne–Beilinson cohomology sheaves H .Z.q/D/ are torsion-free as a consequence of the Bloch–Kato conjectures as proven by Rost 0 qC1 and Voevodsky. This implies that H .X; H .Z.q/D// D 0 if X is unirational. For a surface X with pg D 0 we show that the Albanese kernel, identified with 0 3 H .X; H .Z.2/D//, can be characterized using the integral part of the sheaves associated to the Hodge filtration. Introduction ∗ For a compact algebraic manifold X over C, the Deligne cohomology H .X; Z. · /D/ is defined by taking the hypercohomology of the truncated de Rham complex aug- mented over Z. The extension of such a cohomology theory to arbitrary algebraic complex varieties is usually called Deligne–Beilinson cohomology (for example, see[Gillet 1984; Esnault and Viehweg 1988] for definitions, properties and de- tails). Since Deligne–Beilinson cohomology yields a Poincaré duality theory with ∗ supports, the associated Zariski sheaves H .Z. · /D/ have groups of global sections which are birational invariants of smooth complete varieties (see [Barbieri-Viale 1994; 1997]). The motivation for this paper is to start an investigation of these invariants. qC1 We can show that the Deligne–Beilinson cohomology sheaves H .Z.q/D/ are torsion-free (see Theorem 2.5) as a consequence of the Bloch–Kato isomorphisms, proven by Rost and Voevodsky (for example, see[Haesemeyer and Weibel 2014; 3 Voevodsky 2011; Weibel 2008]). In particular, H .Z.2/D/ is torsion-free thanks to the Merkurjev–Suslin theorem[1983] on K2. Thus, the corresponding invariants vanish for unirational varieties (see Corollary 2.8). Note that also the singular co- homology sheaves Hq .Z/ are torsion-free. Indeed, a conditional proof (depending on the validity of the Bloch–Kato conjectures) of these properties has been known for a long time (see Remark 2.13). 2 Furthermore, if only H .X; OX / D 0, we can show that the group of global sec- 3 2 4 tions of H .Z.2/D/ is exactly the kernel of the cycle map CH .X/ ! H .X; Z.2/D/ in Deligne cohomology, which contains the kernel of the Abel–Jacobi map (see MSC2010: primary 14C35; secondary 14C30, 14F42. Keywords: K -theory, Hodge theory, algebraic cycles. 3 4 LUCABARBIERI-VIALE Proposition 3.3 and Remark 3.6). This fact generalizes the result of H. Esnault [1990a, Theorem 2.5] for 0-cycles in the case of codimension-2 cycles to X of arbitrary dimension, and it is obtained by a different proof. Concerning the dis- 2;2 2 crete part FZ of the Deligne–Beilinson cohomology sheaf H .Z.2/D/, we can 1 2;2 describe, for any X proper and smooth, the torsion of H .X; FZ / in terms of 3 “transcendental cycles” and H .X; Z/tors, and we can see that it has no nonzero global sections (see Proposition 3.3). For surfaces with pg D 0, we are able to 3 compute the group of global sections of H .Z.2/D/ — Bloch’s conjecture is that 0 3 H .X; H .Z.2/D// D 0 — by means of some short exact sequences (described 2;2 in Proposition 4.3) involving the discrete part FZ and the Hodge filtration. In order to do that, we argue first with arithmetic resolutions of the Zariski sheaves associated with the presheaves of mixed Hodge structures defined by singular coho- mology: the Hodge and weight filtrations do have corresponding coniveau spectral sequences, the E2 terms of which are given by the cohomology groups of the Zariski sheaves associated to such filtrations (see Proposition 1.2, Theorem 1.6 and compare with the local Hodge theory of[Barbieri-Viale 2002, §3]). Notation. Throughout this note X is a complex algebraic variety. Let H ∗.X; A/ and H∗.X; A/ be the singular cohomology and Borel–Moore homology of the as- sociated analytic space Xan with coefficients in A, respectively, where A could ∗ ∗ be Z; Z=n; C or C . Let H .X/ and H∗.X/ be the corresponding mixed Hodge ∗ structures, respectively (see[Deligne 1971]). Denote by Wi H .X/ and W−i H∗.X/ i ∗ −i the Q-vector spaces given by the weight filtration, and by F H .X/ and F H∗.X/ the complex vector spaces given by the Hodge filtration, respectively. For the ring Z of integers, we will denote by Z.r/ the Tate twist in Hodge theory and ∗ by H .X; Z.r/D/ the Deligne–Beilinson cohomology groups (see[Esnault and Viehweg 1988; Gillet 1984]). The Tate twist induces the twist A ⊗ Z.r/ in the coefficients. which we shall denote by A.r/ for short. Denote by H∗.A.r// and ∗ H .Z.r/D/ the Zariski sheaves on a given X associated to singular cohomology and Deligne–Beilinson cohomology, respectively. 1. Arithmetic resolutions in mixed Hodge theory Let Z ,! X be a closed subscheme of the complex algebraic variety X. According ∗ to[Deligne 1974, (8.2.2) and (8.3.8)], the singular cohomology groups HZ .X; Z/ carry a mixed Hodge structure fitting into long exact sequences j j j jC1 ···! HZ .X/ ! HT .X/ ! HT −Z .X − Z/ ! HZ .X/ !··· (1.1) for any pair Z ⊂ T of closed subschemes of X.