# Journal of K-theory K-theory and its Applications to Algebra Geometry and Topology

Published by Cambridge University Press (CUP)

Online ISSN: 1865-5394

Print ISSN: 1865-2433

Published by Cambridge University Press (CUP)

Online ISSN: 1865-5394

Print ISSN: 1865-2433

Publications

We study the E-theory group $E_{[0,1]}(A,B)$ for a class of C*-algebras over
the unit interval with finitely many singular points, called elementary
$C[0,1]$-algebras. We use results on E-theory over non-Hausdorff spaces to
describe $E_{[0,1]}(A,B)$ where $A$ is a sky-scraper algebra. Then we compute
$E_{[0,1]}(A,B)$ for two elementary $C[0,1]$-algebras in the case where the
fibers $A(x)$ and $B(y)$ of $A$ and $B$ are such that $E^1(A(x),B(y)) = 0$ for
all $x,y\in [0,1]$. This result applies whenever the fibers satisfy the UCT,
their $K_0$-groups are torsion-free and their $K_1$-groups are zero. In that
case we show that $E_{[0,1]}(A,B)$ is isomorphic to $\hombbk{A}{B}$, the group
of morphisms of the K-theory sheaves of $A$ and $B$.

…

We define generalized Atiyah-Patodi-Singer boundary conditions of product type for Dirac operators associated to C*-vector bundles on the product of a compact manifold with boundary and a closed manifold. We prove a product formula for the K-theoretic index classes, which we use to generalize the product formula for the topological signature to higher signatures.

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Starting from the candidate Bloch-Beilinson filtration on Chow groups of 0-cycles constructed by J. Lewis, we develop and describe geometrically a series of Hodge-theoretic invariants defined on the graded pieces. Explicit formulas (in terms of currents and membrane integrals) are given for certain quotients of the invariants, with applications to 0-cycles on products of curves.

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We study Quillen's model category structure for homotopy of simplicial objects in the context of Janelidze, Marki and Tholen's semi-abelian categories. This model structure exists as soon as the base category A is regular Mal'tsev and has enough regular projectives; then the fibrations are the Kan fibrations of simplicial objects in A. When, moreover, A is semi-abelian, weak equivalences and homology isomorphisms coincide. Comment: 12 pages

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In this paper we suggest a definition for the category of mixed motives
generated by the motive h^1(E) for E an elliptic curve without complex
multiplication. We then compute the cohomology of this category. Modulo a
strengthening of the Beilinson-Soule conjecture, we show that the cohomology of
our category agrees with the expected motivic cohomology groups. Finally for
each pure motive (Sym^{n}h^1(E))(-1) we construct families of nontrivial
motives whose highest associated weight graded piece is $Sym^{n}h^1(E))(-1).
This paper was essentially written in the late 1990's whilst the author was
at the University of Chicago. The author apologizes for the tardiness of this
posting, and hopes the reader will still find the content interesting.

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In their 2007 paper, Jarvis, Kaufmann, and Kimura defined the full orbifold $K$-theory of an orbifold ${\mathfrak X}$, analogous to the Chen-Ruan orbifold cohomology of ${\mathfrak X}$ in that it uses the obstruction bundle as a quantum correction to the multiplicative structure. We give an explicit algorithm for the computation of this orbifold invariant in the case when ${\mathfrak X}$ arises as an abelian symplectic quotient. Our methods are integral $K$-theoretic analogues of those used in the orbifold cohomology case by Goldin, Holm, and Knutson in 2005. We rely on the $K$-theoretic Kirwan surjectivity methods developed by Harada and Landweber. As a worked example, we present the full computation of the full orbifold $K$-theory of weighted projective spaces. Our computations hold over the integers, and in the particular case of weighted projective spaces, we show that the associated invariant is torsion-free.

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The Bost conjecture with C*-algebra coefficients for locally compact Hausdorff groups passes to open subgroups. We also prove that if a locally compact Hausdorff group acts on a tree, then the Bost conjecture with C*-coefficients is true for the group if and only if it is true for the stabilisers of the vertices.

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In this paper we define complex equivariant K-theory for actions of Lie
groupoids using finite-dimensional vector bundles. For a Bredon-compatible Lie
groupoid, this defines a periodic cohomology theory on the category of finite
equivariant CW-complexes. We also establish an analogue of the completion
theorem of Atiyah and Segal. Some examples are discussed.

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The additive invariants of an algebraic variety is calculated in terms of those of the fixed point set under the action of additive and multiplicative groups, by using Bialynicki-Birula's fixed point formula for a projective algebraicset with a G_m-action or G_a-action. The method is also generalized to calculate certain additive invariants for Chow varieties. As applications, we obtain the Hodge polynomial of Chow varieties in characteristic zero and the number of points for Chow varieties over finite fields. As applications, we obtain the l-adic Euler-Poincare characteristic for the Chow varieties of certain projective varieties over an algebraically closed field of arbitrary characteristic. Moreover, we show that the virtual Hodge (p,0) and (0,q)-numbers of the Chow varieties and affine group varieties are zero for all p,q positive. Comment: 16 pages, Title changed, more additive invariants are considered

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We develop equivariant KK-theory for locally compact groupoid actions by
Morita equivalences on real and complex graded C*-algebras. Functoriality with
respect to generalised morphisms and Bott periodicity are discussed. We
introduce Stiefel-Whitney classes for real or complex equivariant vector
bundles over locally compact groupoids to establish the Thom isomorphism
theorem in twisted groupoid K-theory.

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We describe the dualization of the algebra of secondary cohomology operations in terms of generators extending the Milnor dual of the Steenrod algebra. In this way we obtain explicit formulae for the computation of the E_3-term of the Adams spectral sequence converging to the stable homotopy groups of spheres.

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For any regular noetherian scheme X and every k>0, we define a chain morphism between two chain complexes whose homology with rational coefficients is isomorphic to the algebraic K-groups of X tensored by the field of rational numbers. It is shown that these morphisms induce in homology the Adams operations defined by Gillet and Soule or the ones defined by Grayson.

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A new model of smooth K ⁰ -theory ([5] [1]) is constructed, with the help of the total Chern class (contrary to the theories considered in ]1], [5], [12] and [13] which use the Chern character). The correspondence with the earlier model [1] is obtained by adapting, at the level of transgression forms, the usual formulae which express the Chern character in terms of the Chern classes and vice versa.
The advantage of this new model is that it allows constructing Chern classes with values in integral Chern-Simons characters in a natural way: this construction answers a question asked by U. Bunke [4].

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There are two infinitesimal (i.e., additive) versions of the K -theory of a field F : one introduced by Cathelineau, which is an F -module, and the other introduced by Bloch-Esnault, which is an F *-module. Both versions are equipped with a regulator map, when F is the field of complex numbers.
We will introduce an extended version of Cathelineau's group, and a complex-valued regulator map given by the entropy. We will also give a comparison map between our extended version and Cathelineau's group.
Our results were motivated by two unrelated sources: Neumann's work on the extended Bloch group (which is isomorphic to indecomposable K 3 of the complex numbers), and the study of singularities of generating series of hypergeometric multisums.

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Motivated by traces of matrices and Euler characteristics of topological spaces, we expect abstract traces in a symmetric monoidal category to be “additive”. When the category is “stable” in some sense, additivity along cofiber sequences is a question about the interaction of stability and the monoidal structure.
May proved such an additivity theorem when the stable structure is a triangulation, based on new axioms for monoidal triangulated categories. in this paper we use stable derivators instead, which are a different model for “stable homotopy theories”. We define and study monoidal structures on derivators, providing a context to describe the interplay between stability and monoidal structure using only ordinary category theory and universal properties. We can then perform May's proof of the additivity of traces in a closed monoidal stable derivator without needing extra axioms, as all the needed compatibility is automatic.

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We provide a complete analysis of the motivic Adams spectral sequences
converging to the bigraded coefficients of the 2-complete algebraic
Johnson-Wilson spectra BPGL over p-adic fields. These spectra interpolate
between integral motivic cohomology (n=0), a connective version of algebraic
K-theory (n=1), and the algebraic Brown-Peterson spectrum. We deduce that, over
p-adic fields, the 2-complete BPGL split over 2-complete BPGL<0>, implying
that the slice spectral sequence for BPGL collapses.
This is the first in a series of two papers investigating motivic invariants
of p-adic fields, and it lays the groundwork for an understanding of the
motivic Adams-Novikov spectral sequence over such base fields.

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We derive a power series formula for the $p$-adic regulator on the higher dimensional algebraic K-groups of number fields. This formula is designed to be well suited to computer calculations and to reduction modulo powers of $p$. In addition we describe a series of regulator questions concerning higher dimensional K-theoretic analogues of conjectures of Gross and Serre.

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Let A be an affine algebra of dimension n over an algebraically closed field k with 1/ n ! ∈ k . Let P be a projective A -module of rank n − 1. Then, it is an open question due to N. Mohan Kumar, whether P is cancellative. We prove the following results:
( i ) If A = R [ T , T ⁻¹ ], then P is cancellative.
( ii ) If A = R [ T ,1/ f ] or A = R [ T , f 1 / f ,…, f r / f ], where f ( T ) is a monic polynomial and f , f 1 ,…, f r is R [ T ]-regular sequence, then A n −1 is cancellative. Further, if k = p , then P is cancellative.

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We prove that the embedding of the derived category of 1-motives into the
triangulated category of effective Voevodsky motives, as well as its left
adjoint functor $LAlb$, commute with the Hodge realization.

…

Suppose that $B$ is a $G$-Banach algebra over $\mathbb{F} = \mathbb{R}$ or
$\mathbb{C}$, $X$ is a finite dimensional compact metric space, $\zeta : P \to
X$ is a standard principal $G$-bundle, and $A_\zeta = \Gamma (X, P \times_G B)$
is the associated algebra of sections.
We produce a spectral sequence which converges to $\pi_*(GL_o A_\zeta) $ with
[E^2_{-p,q} \cong \check{H}^p(X ; \pi_q(GL_o B)).] A related spectral sequence
converging to $\K_{*+1}(A_\zeta)$ (the real or complex topological $K$-theory)
allows us to conclude that if $B$ is Bott-stable, (i.e., if $ \pi_*(GL_o B) \to
\K_{*+1}(B)$ is an isomorphism for all $*>0$) then so is $A_\zeta$.

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This paper presents, by example, an index theory appropriate to algebras without trace. Whilst we work exclusively with the Cuntz algebras the exposition is designed to indicate how to develop a general theory. Our main result is an index theorem (formulated in terms of spectral flow) using a twisted cyclic cocycle where the twisting comes from the modular automorphism group for the canonical gauge action on the Cuntz algebra. We introduce a modified $K_1$-group of the Cuntz algebra so as to pair with this twisted cocycle. As a corollary we obtain a noncommutative geometry interpretation for Araki's notion of relative entropy in this example. We also note the connection of this example to the theory of noncommutative manifolds.

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In this paper we extend the computation of the the typical curves of algebraic K-theory done by Lars Hesselholt and Ib Madsen to general tensor algebras. The models used allow us to determine the stages of the Taylor tower of algebraic K-theory as a functor of augmented algebras, as defined by Tom Goodwillie, when evaluated on derived tensor algebras.For R a discrete ring, and M a simplicial R-bimodule, we let
R
(M) denote the (derived) tensor algebra of M over R, and
π
R
denote the ring of formal (derived) power series in M over R. We define a natural transformation of functors of simplicial R-bimodules Φ: which is closely related to Waldhausen's equivalence We show that Φ induces an equivalence on any finite stage of Goodwillie's Taylor towers of the functors at any simplicial bimodule. This is used to show that there is an equivalence of functors , and for connected bimodules, also an equivalence

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Based on the algebraic cobordism theory of Levine and Morel, we develop a
theory of algebraic cobordism modulo algebraic equivalence.
We prove that this theory can reproduce Chow groups modulo algebraic
equivalence and the semi-topological $K_0$-groups. We also show that with
finite coefficients, this theory agrees with the algebraic cobordism theory.
We compute our cobordism theory for some low dimensional varieties. The
results on infinite generation of some Griffiths groups by Clemens and on
smash-nilpotence by Voevodsky and Voisin are also lifted and reinterpreted in
terms of this cobordism theory.

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We consider the algebraic K-theory of a truncated polynomial algebra in several commuting variables, . This naturally leads to a new generalization of the big Witt vectors. If k is a perfect field of positive characteristic we describe the K-theory computation in terms of a cube of these Witt vectors on ℕ
n
. If the characteristic of k does not divide any of the ai
we compute the K-groups explicitly. We also compute the K-groups modulo torsion for k = ℤ.To understand this K-theory spectrum we use the cyclotomic trace map to topological cyclic homology, and write as the iterated homotopy cofiber of an n-cube of spectra, each of which is easier to understand.

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For every regular cardinal α, we construct a cofibrantly generated Quillen model structure on a category whose objects are essentially dg categories which are stable under suspensions, cosuspensions, cones and α-small sums.
Using results of Porta, we show that the category of well-generated (algebraic) triangulated categories in the sense of Neeman is naturally enhanced by our Quillen model category.

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Under the assumption that the base field k has characteristic 0, we prove a formula for the push-forward class of Bott-Samelson resolutions in the algebraic cobordism ring of the flag bundle. We specialise our formula to connective K-theory providing a geometric interpretation to the double β-polynomials of Fomin and Kirillov by computing the fundamental classes of schubert varieties. As a corollary we obtain a Thom-Porteous formula generalising those of the Chow ring and of the Grothendieck ring of vector bundles.

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This paper presents a proof of reciprocity laws for the Parshin symbol and
for two new local symbols, defined here, which we call 4-function local
symbols. The reciprocity laws for the Parshin symbol are proven using a new
method - via iterated integrals. The usefulness of this method is shown by two
facts - first, by establishing new local symbols - the 4-function local symbols
and their reciprocity laws and, second, by providing refinements of the Parshin
symbol in terms of bi-local symbols, each of which satisfies a reciprocity law.
The K-theoretic variant of the first 4-function local symbol is defined in the
Appendix. It differs by a sign from the one defined via iterated integrals.
Both the sign and the K-theoretic variant of the 4-function local symbol
satisfy reciprocity laws.

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We examine the theory of connective algebraic K-theory, , defined by taking the −1 connective cover of algebraic K-theory with respect to Voevodsky's slice tower in the motivic stable homotopy category. We extend to a bi-graded oriented duality theory when the base scheme is the spectrum of a field k of characteristic zero. The homology theory may be viewed as connective algebraic G-theory. We identify for X a finite type k-scheme with the image of in , where is the abelian category of coherent sheaves on X with support in dimension at most n; this agrees with the (2n,n) part of the theory of connective algebraic K-theory defined by Cai. We also show that the classifying map from algebraic cobordism identifies with the universal oriented Borel-Moore homology theory having formal group law u + υ − βuυ with coefficient ring ℤ[β]. As an application, we show that every pure dimension d finite type k-scheme has a well-defined fundamental class [X]
CK
in Ω
d
CK
(X), and this fundamental class is functorial with respect to pull-back for l.c.i. morphisms.

…

In his 1973 paper Quillen proved a resolution theorem for the K-Theory of an
exact category; his proof was homotopic in nature. By using the main result of
a paper by Nenashev, we are able to give an algebraic proof of Quillen's
Resolution Theorem for K_1 of an exact category.

…

In the world of chain complexes E_n-algebras are the analogues of based
n-fold loop spaces in the category of topological spaces. Fresse showed that
operadic E_n-homology of an E_n-algebra computes the homology of an n-fold
algebraic delooping. The aim of this paper is to construct two spectral
sequences for calculating these homology groups and to treat some concrete
classes of examples such as Hochschild cochains, graded polynomial algebras and
chains on iterated loop spaces. In characteristic zero we gain an
identification of the summands in Pirashvili's Hodge decomposition of higher
order Hochschild homology in terms of derived functors of indecomposables of
Gerstenhaber algebras and as the homology of exterior and symmetric powers of
derived K\"ahler differentials.

…

We give a universal construction of a derived affine group scheme and its
representation category from a symmetric monoidal infinity-category, which we
shall call the tannnakization of a symmetric monoidal infinity-category. This
can be viewed as infinity-categorical generalization of the work of
Joyal-Street and Nori. We then apply it to the stable infinity-category of
mixed motives equipped with the realization functor of a mixed Weil cohomology
and obtain a derived motivic Galois group whose representation category has a
universality, and which represents the automorphism group of the realization
functor. Also, we present basic properties of derived affine group schemes in
Appendix.

…

Hazrat gave a K-theoretic invariant for Leavitt
path algebras as graded algebras. Hazrat conjectured that this
invariant classifies Leavitt path algebras up to graded isomorphism,
and proved the conjecture in some cases. In this paper, we prove
that a weak version of the conjecture holds for all finite graphs with neither sources
nor sinks.

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The purpose of this paper is to extend our knowledge when a pair A, H where H is a Hopf algebra over a field and A is a right coideal subalgebra has the property that H is either projective or flat as an A-module. The projectivity is obtained under the assumption that H is residually finite dimensional, A has semilocal localizations with respect to a central subring, and there exists a Hopf subalgebra B of H such that the antipode of B is bijective and B is a finitely generated A-module. The flatness of H over A is shown to hold when H is a directed union of residually finite dimensional Hopf subalgebras, and there exists a Hopf subalgebra of H whose center contains A. More general projectivity and flatness results are established for (co)equivariant modules over an H-(co)module algebra under similar assumptions.

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In this paper, we study Poisson (co)homology of a Frobenius Poisson algebra. More precisely, we show that there exists a duality between Poisson homology and Poisson cohomology of Frobenius Poisson algebras, similar to that between Hochschild homology and Hochschild cohomology of Frobenius algebras. Then we use the non-degenerate bilinear form on a unimodular Frobenius Poisson algebra to construct a Batalin-Vilkovisky structure on the Poisson cohomology ring making it into a Batalin-Vilkovisky algebra.

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This paper is comprised of two related parts. First we discuss which k -graph algebras have faithful traces. We characterise the existence of a faithful semifinite lower-semicontinuous gauge-invariant trace on C * (Λ) in terms of the existence of a faithful graph trace on Λ.
Second, for k -graphs with faithful gauge invariant trace, we construct a smooth ( k ,∞)-summable semifinite spectral triple. We use the semifinite local index theorem to compute the pairing with K -theory. This numerical pairing can be obtained by applying the trace to a KK -pairing with values in the K -theory of the fixed point algebra of the T k action. As with graph algebras, the index pairing is an invariant for a finer structure than the isomorphism class of the algebra.

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This paper is concerned with the theory of cup products in the Hopf cyclic cohomology of algebras and coalgebras. We show that the cyclic cohomology of a coalgebra can be obtained from a construction involving the noncommutative Weil algebra. Then we introduce the notion of higher -twisted traces and use a generalization of the Quillen and Crainic constructions (see [14] and [3]) to define the cup product. We discuss the relation of the cup product above and S -operations on cyclic cohomology. We show that the product we define can be realized as a combination of the composition product in bivariant cyclic cohomology and a map from the cyclic cohomology of coalgebras to bivariant cohomology. In the last section, we briefly discuss the relation of our constructions with that in [9]. More precisely, we propose still another construction of such pairings which can be regarded as an intermediate step between the “Crainic” pairing and that of [9]. We show that it coincides with what in [9] and as far its relation to Crainic's construction is concerned, we reduce the question to a discussuion of a certain map in cohomology (see the question at the end of section 5).
The results of the current paper were announced in [12].

…

Let $C^*(E)$ be the graph $C^*$-algebra associated to a graph E and let J be
a gauge invariant ideal in $C^*(E)$. We compute the cyclic six-term exact
sequence in $K$-theory of the associated extension in terms of the adjacency
matrix associated to $E$. The ordered six-term exact sequence is a complete
stable isomorphism invariant for several classes of graph $C^*$-algebras, for
instance those containing a unique proper nontrivial ideal. Further, in many
other cases, infinite collections of such sequences comprise complete
invariants. Our results allow for explicit computation of the invariant, giving
an exact sequence in terms of kernels and cokernels of matrices determined by
the vertex matrix of $E$.

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In this work we consider deformations of Leibniz algebras over a field of characteristic zero. The main problem in deformation theory is to describe all non-equivalent deformations of a given object. We give a method to solve this problem completely, namely work out a construction of a versal deformation for a given Leibniz algebra, which induces all non-equivalent deformations and is unique on the infinitesimal level.

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We show that filtered K-theory is equivalent to a substantially smaller
invariant for all real-rank-zero C*-algebras with certain primitive ideal
spaces -- including the infinitely many so-called accordion spaces for which
filtered K-theory is known to be a complete invariant. As a consequence, we
give a characterization of purely infinite Cuntz-Krieger algebras whose
primitive ideal space is an accordion space.

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We develop a suitable version of the stable module category of a finite group
G over an arbitrary commutative ring k. The purpose of the construction is to
produce a compactly generated triangulated category whose compact objects are
the finitely presented kG-modules. The main idea is to form a localisation of
the usual version of the stable module category with respect to the filtered
colimits of weakly injective modules. There is also an analogous version of the
homotopy category of weakly injective kG-modules and a recollement relating the
stable category, the homotopy category, and the derived category of kG-modules.

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Let A and B be separable nuclear continuous C(X)-algebras over a finite dimensional compact metrizable space X. It is shown that an element $\sigma$ of the parametrized Kasparov group KK_X(A,B) is invertible if and only if all its fiberwise components $\sigma_x\in KK(A(x),B(x))$ are invertible. This criterion does not extend to infinite dimensional spaces since there exist nontrivial unital separable continuous fields over the Hilbert cube with all fibers isomorphic to the Cuntz algebra O_2. Several applications to continuous fields of Kirchberg algebras are given. It is also shown that if each fiber of a separable nuclear continuous C(X)-algebra A over a finite dimensional locally compact space X satisfies the UCT, then A satisfies the UCT.

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C*-algebras A of compact operators are characterized as those C*-algebras of coefficients of Hilbert C*-modules for which (i) every bounded A-linear operator between two Hilbert A-modules possesses an adjoint operator, (ii) the kernels of all bounded A-linear operators between Hilbert A-modules are orthogonal summands, (iii) the images of all bounded A-linear operators with closed range between Hilbert A-modules are orthogonal summands, and (iv) for every Hilbert A-module every Hilbert A-submodule is a topological summand. Thus, the theory of Hilbert C*-modules over C*-algebras of compact operators has similarities with the theory of Hilbert spaces. In passing, we obtain a general closed graph theorem for bounded module operators on arbitrary Hilbert C*-modules.

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We prove that uniform Roe C*-algebras C
*
u
X associated to some expander graphs X coming from discrete groups with property (τ) are not K-exact. In particular, we show that this is the case for the expander obtained as Cayley graphs of a sequence of alternating groups (with appropriately chosen generating sets).

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We study the K-theory of ring C*-algebras associated to rings of integers in
global function fields with only one single infinite place. First, we compute
the torsion-free part of the K-groups of these ring C*-algebras. Secondly, we
show that, under a certain primeness condition, the torsion part of K-theory
determines the inertia degrees at infinity of our function fields.

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Using the classical universal coefficient theorem of Rosenberg-Schochet, we
prove a simple classification of all localizing subcategories of the Bootstrap
category of separable complex C*-algebras. Namely, they are in bijective
correspondence with subsets of the Zariski spectrum of the integers --
precisely as for the localizing subcategories of the derived category of
complexes of abelian groups. We provide corollaries of this fact and put it in
context with similar classifications available in the literature.

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In previous papers (arxiv:math/0612370 and arxiv:0909.1342) we defined the C*-algebra and the longitudinal pseudodifferential calculus of any singular foliation (M,F). Here we construct the analytic index of an elliptic operator as a KK-theory element, and prove that the same element can be obtained from an "adiabatic foliation" TF on $M \times \R$, which we introduce here. Comment: 19 pages. Correction of typos in v2, v3.

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This an extended version of the previous preprint dated by February 2005. We prove that the Chow motive of an anisotropic projective homogeneous variety of type F4 is isomorphic to the direct sum of twisted copies of a generalized Rost motive. In particular, we provide an explicit construction of a generalized Rost motive for a generically splitting variety for a symbol in K_3^M(k)/3. We also establish a motivic isomorphism between two anisotropic non-isomorphic projective homogeneous varieties of type F4. All our results hold for Chow motives with integral coefficients. Comment: 20 pages, XYPIC

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Let S be a degree six del Pezzo surface over an arbitrary field F . Motivated by the first author's classification of all such S up to isomorphism [3] in terms of a separable F -algebra B × Q × F , and by his K-theory isomorphism K n ( S ) ≅ K n ( B × Q × F ) for n ≥ 0, we prove an equivalence of derived categories
where A is an explicitly given finite dimensional F -algebra whose semisimple part is B × Q × F .

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We prove a formula for the cup product on the l-adic cohomology of the complement of a linear subspace arrangement.

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This paper studies Auslander-Reiten triangles in subcategories of triangulated categories. The main theorem shows that the Auslander-Reiten triangles in a subcategory are closely connected with the approximation properties of the subcategory. Namely, let C be an object in the subcategory C of the triangulated category T, and let
be an Auslander-Reiten triangle in T. Then under suitable assumptions, there is an Auslander-Reiten triangle
in C if and only if there is a minimal right-C-approximation of the form
.
The theory is used to give a new proof of the existence of Auslander-Reiten sequences over finite dimensional algebras.

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