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R. Ji, C. Ogle, and B. Ramsey. Relatively Hyperbolic

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R. Ji, C. Ogle, and B. Ramsey. Relatively Hyperbolic J. Noncommut. Geom. 4 (2010), 83–124 Journal of Noncommutative Geometry DOI 10.4171/JNCG/50 © European Mathematical Society Relatively hyperbolic groups, rapid decay algebras and a generalization of the Bass conjecture Ronghui Ji, Crichton Ogle, and Bobby Ramsey With an Appendix by Crichton Ogle Abstract. By deploying dense subalgebras of `1.G/ we generalize the Bass conjecture in terms of Connes’ cyclic homology theory. In particular, we propose a stronger version of the `1-Bass Conjecture. We prove that hyperbolic groups relative to finitely many subgroups, each of which posses the polynomial conjugacy bound property and nilpotent periodicity property, satisfy the `1-Stronger-Bass Conjecture. Moreover, we determine the conjugacy bound for relatively hyperbolic groups and compute the cyclic cohomology of the `1-algebra of any discrete group. Mathematics Subject Classification (2010). 46L80, 20F65, 16S34. Keywords. B-bounded cohomology, rapid decay algebras, Bass conjecture. Introduction Let R be a discrete ring with unit. A classical construction of Hattori–Stallings ([Ha], [St]) yields a homomorphism of abelian groups HS a Tr K0 .R/ ! HH0.R/; a where K0 .R/ denotes the zeroth algebraic K-group of R and HH0.R/ D R=ŒR; R its zeroth Hochschild homology group. Precisely, given a projective module P over R, one chooses a free R-module Rn containing P as a direct summand, resulting in a map n Trace EndR.P / ,! EndR.R / D Mn;n.R/ ! HH0.R/: HS One then verifies the equivalence class of Tr .ŒP / ´ Trace..IdP // in HH0.R/ depends only on the isomorphism class of P , is invariant under stabilization, and sends direct sums to sums. Hence it extends uniquely to a map on K0.R/,asabove. Let G be a discrete group, k a commutative ring and kŒG the group algebra of G with coefficients in k  C. Let hGi denoteL the set of conjugacy classesL of G. Taking tensor products over k,HH0.kŒG/ D x2hGi HH0.kŒG/x D x2hGi k 84 R. Ji, C. Ogle, and B. Ramsey decomposes as a direct sum of copies of k indexed on hGi. Let x W HH0.kŒG/ ! k be the projection which is the identity on the summand indexed by x and sends the other summands to zero. Given a finitely generated projective module P over kŒG, HS the P -rank of an element g 2 G is given by rP .g/ ´ hgi Tr .ŒP /. By convention, the conjugacy classes associated to elements of finite order are referred to as elliptic, and the sum of all such the elliptic summand. In [Ba], H. Bass stated the following conjecture. Bass Conjecture. If P is a finitely generated projective module over ZŒG, then rP .g/ D 0 if g ¤ 1. Although still unknown in general, many partial results are known, beginning with the case G a linear group, verified by Bass himself in [Ba]. For the integral group algebra, Linell has shown that rP .g/ D 0 whenever 1 ¤Lg is of finite order. Also, the Z C Z Z C Linclusion ,! induces an inclusion HH0. ŒG/ D x2hGi ,! HH0. ŒG/ D C C C x2hGi (where HH0. ŒG/ is computed with tensors over ), which is the obvious inclusion on each summand. Thus the Bass conjecture is implied by the Strong Bass Conjecture – SBC. For each non-elliptic class x, the image of the HS a C C C composition x B Tr W K0 . ŒG/ ! HH0. ŒG/ HH0. ŒG/x is zero. Henceforth all algebras and tensor products are assumed to be over C.IfA is a Fréchet algebra and Aı denotes A with the discrete topology, then there is a commuting diagram HS a ı Tr / ı K0 .A / HH0.A / t ch0 / t K0.A/ HC0.A/, where the vertical maps are induced by the continuous map Aı ! A which is the identity on elements, ch0 is the Connes–Karoubi–Chern character in dimension zero t [CK], and HC0.A/ the zeroth (reduced topological) cyclic homology group of A. By [Ti], this same diagram exists for arbitrary topological algebras A when A is a topologized by the fine topology, in which case one has an equality HC.A/ D t HC.A/. The advantage to working with ch is that it fits into a family of Chern characters m t t chn W Kn.A/ ! HCnC2m.A/ m m1 t which satisfy the identity S B chn D chn for m 1, where S W HCnC2m.A/ ! t HCnC2.m1/.A/ is the S operator. Moreover, when A is a (locally convex) algebra t topologized with the fine topology, there is an isomorphism HC.A/ D HC.A/, with the left-hand side denoting algebraic cyclic homology. In particular, we can take A D CŒG equipped with the fine topology. Then for each conjugacy class x, Hyperbolic groups, decay algebras and a generalization of the Bass conjecture 85 t there is a summand HC.CŒG/x of HC.CŒG/ D HC.CŒG/ and a corresponding projection operator .x/ W HC.CŒG/ HC.CŒG/x. In light of this, it is natural to consider the following stronger version of SBC. Stronger Bass Conjecture – SrBC. For each non-elliptic class x, the image of the t composition .x/ B ch W K.CŒG/ ! HC.CŒG/ ! HC.CŒG/x is zero. t It follows by [Og1] that the image of ch W K.CŒG/ ! HC.CŒG/ is at least the elliptic summand; the non-trivial part of the conjecture is to show that it is no more than this. For a given non-elliptic class x, we say that x satisfies the nilpotency condition C2 if the operator S W HC .CŒG/x ! HC .CŒG/x is nilpotent. We say that G satisfies the nilpotency condition if x does for each non-elliptic conjugacy class x. Combining the results of Connes–Karoubi with those of Burghelea, one has the following immediate consequence, used in [Ec1], [Ji3] and [Em]: Observation. Let x be a non-elliptic conjugacy class satisfying the nilpotency con- x dition. Then the composition Kn.CŒG/ ! HCn.CŒG/ ! HCn.CŒG/x is zero for all n 0. Consequently, if G satisfies the nilpotency condition, then the SrBC is true for G. Unlike SrBC, there are known examples of discrete groups which do not satisfy the nilpotency condition. However, there is a large class of groups which do. The most inclusive results are those of Emmanouil [Em]. Following [Em] we denote by C.C/ the class of groups which satisfy the nilpotency condition over C (this is the same as over Q, but the notation is more in keeping with the rest of our paper). Let `1.G/ denote the `1-algebra of G. Fixing a proper word-length function L 1;1 1 on G, we denote by HL .G/ the standard ` -rapid-decay-algebra of G; this is a Fréchet subalgebra of `1.G/ containing CŒG which is smooth in `1.G/, that is, the 1;1 1 inclusion HL .G/ ,! ` .G/ induces an isomorphism on topological K-groups in all degrees [Jo1], [Jo2]. The Connes–Karoubi–Chern character therefore produces a map t 1 Š t 1;1 t 1;1 K.` .G// K.HL .G// ! HC.HL .G//: Analogous to the case of the group algebra, there is a naturally defined collec- t 1;1 t 1;1 t 1;1 tion of summands fHH.HL .G//xg resp. fHC.HL .G//xg of HH.HL .G// t 1;1 resp. HC.HL .G// indexed on the set of conjugacy classes of G, with a similar result for topological Hochschild and cyclic cohomology. This leads to the following variant of SrBC, which was one of our original motivations for this paper. `1-SrBC. For each non-elliptic conjugacy class x, the image of the composition t 1 t 1;1 t 1;1 .x/ B ch W K.` .G// ! HC.HL .G// HC.HL .G//x is zero. 86 R. Ji, C. Ogle, and B. Ramsey It is the content of the `1-Bass Conjecture that the `1-Hattors–Stallings trace 1 1 1 HS W K0.` .G// ! HH0.` .G// has a range concentrated in elliptic summands. In [BCM] it is shown that the Baum–Connes Conjecture for a discrete group implies its `1-Bass Conjecture. Thus, a-T-menable groups, in particular, amenable groups satisfy the `1-Bass conjecture, following the work of Higson and Kasparov [HK]. The `1-SrBC we have proposed here is stronger than the `1-Bass Conjecture. Our method requires determining the nilpotency of the periodicity operator on cyclic cohomology and some geometric analysis on the group. However, it does not require the verification of Baum–Connes conjecture. An outline of the paper is as follows. In Section 1.2, following a preliminary review of Hochschild, cyclic and periodic cyclic (co)homology, we introduce the notion of a bounding function. A family B of such functions, together with a word- length function L on G, equips CŒG with a collection of semi-norms, for which the 1 completion HB;L.G/ of CŒG is a subalgebra of ` .G/. In this paper the term rapid decay algebra refers to any subalgebra of `1.G/ of this type. In fact, the rapid decay 1;1 P algebra HL .G/ occurs as HP ;L.G/, where is the family of polynomial bounding functions. The category B of all families of bounding functions is a poset, with par- tial ordering given by inclusion. This category has an initial object Bmin, a terminal object Bmax, and for all .G; L/ (meaning a discrete group G equipped with a word- 1 C length function L) there are equalities HBmin;L.G/ D ` .G/, HBmax;L.G/ D ŒG.
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