Algebraic K-theory of completed Kac-Moody groups
We extend results for the K-theory of Hecke algebras of reductive $p$-adic groups to completed Kac-Moody groups.
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arXiv subjects
Arthur Bartels
Publications and source records attributed to Arthur Bartels.
Almost equivariant maps for td-groups
We construct certain maps from buildings associated to td-groups to a space closely related to the classifying numerable $G$-space for the family $\mathcal{C}$vcy of covirtually cyclic subgroups. These maps are used in forthcoming paper to study the K-theory of Hecke algebras in the spirit of the Farrell-Jones conjecture.
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Algebraic K-theory of reductive p-adic groups
Motivated by the Farrell-Jones Conjecture for group rings, we formulate the $\mathcal{C}$op-Farrell-Jones Conjecture for the K-theory of Hecke algebras of td-groups. We prove this conjecture for (closed subgroups of) reductive p-adic groups G. In particular, the projective class group $K_0(\mathcal{H}(G))$ for a (closed subgroup) of a reductive p-adic group G can be computed as a colimit of projective class groups $K_0(\mathcal{H}(U))$ where U varies over the compact open subgroups of G. This implies that all finitely generated smooth complex representations of a reductive p-adic G admit finite projective resolutions by compactly induced representations. For SL$_n(F)$ we translate the colimit formula for $K_0(\mathcal{H}(G))$ to a more concrete cokernel description in terms of stabilizers for the action on the Bruhat-Tits building. For negative K-theory we obtain vanishing results, while we identify the higher K-groups $K_n(\mathcal{H}(G))$ with the value of G-homology theory on the extended Bruhat-Tits building. Our considerations apply to general Hecke algebras of the form $\mathcal{H}(G;R,ρ,ω)$, where we allow a central character $ω$ and a twist by an action $ρ$ of G on R. For the $\mathcal{C}$op-Farrell-Jones Conjecture we need to assume $\mathbb{Q} \subseteq R$ and a regularity assumption. As a key intermediate step we introduce the $\mathcal{C}vcy-Farrell-Jones conjecture. For the latter no regularity assumptions on R are needed.
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Inheritance properties of the Farrell-Jones Conjecture for totally disconnected groups
In this paper we formulate and lay the foundations for the K-theoretic Farrell-Jones Conjecture for the Hecke algebra of totally disconnected groups. The main result of his paper is the proof that it passes to closed subgroups. Moreover, we carry out some constructions such as the diagonal tensor product and prove some results that will be used in the actual proof of the Farrell-Jones Conjecture for reductive p-adic groups, which will appear in a different paper.
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Recipes to compute the algebraic K-theory of Hecke algebras of reductive p-adic groups
We compute the algebraic K-theory of the Hecke algebra of a reductive p-adic group G using the fact that the Farrell-Jones Conjecture is known in this context. The main tool will be the properties of the associated Bruhat-Tits building and an equivariant Atiyah-Hirzebruch spectral sequence. In particular the projective class group can be written as the colimit of the projective class groups of the compact open subgroups of G.
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On the algebraic K-theory of Hecke algebras
Consider a totally disconnected group G, which is covirtually cyclic, i.e., contains a normal compact open subgroup L such that G/L is infinite cyclic. We establish a Wang sequence, which computes the algebraic K-groups of the Hecke algebra of G in terms of the one of L, and show that all negative K-groups vanish. This confirms the K-theoretic Farrell-Jones Conjecture for the Hecke algebra of G in this special case. Our ultimate long term goal is to prove it for any closed subgroup of any reductive p-adic group. The results of this paper will play a role in the final proof.
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Vanishing of Nil-terms and negative K-theory for additive categories
We extend the notion of regular coherence from rings to additive categories and show that well-known consequences of regular coherence for rings also apply to additive categories. For instance the negative K-groups and all twisted Nil-groups vanish for an additive category if it is regular coherent. This will be applied to nested sequences of additive categories, motivated by our ongoing project to determine the algebraic K-theory of the Hecke algebra of a reductive p-adic group.
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Conformal nets III: fusion of defects
Conformal nets provides a mathematical model for conformal field theory. We define a notion of defect between conformal nets, formalizing the idea of an interaction between two conformal field theories. We introduce an operation of fusion of defects, and prove that the fusion of two defects is again a defect, provided the fusion occurs over a conformal net of finite index. There is a notion of sector (or bimodule) between two defects, and operations of horizontal and vertical fusion of such sectors. Our most difficult technical result is that the horizontal fusion of the vacuum sectors of two defects is isomorphic to the vacuum sector of the fused defect. Equipped with this isomorphism, we construct the basic interchange isomorphism between the horizontal fusion of two vertical fusions and the vertical fusion of two horizontal fusions of sectors.
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Conformal nets IV: The 3-category
Conformal nets are a mathematical model for conformal field theory, and defects between conformal nets are a model for an interaction or phase transition between two conformal field theories. In the preceding paper of this series, we introduced a notion of composition, called fusion, between defects. We also described a notion of sectors between defects, modeling an interaction among or transformation between phase transitions, and defined fusion composition operations for sectors. In this paper we prove that altogether the collection of conformal nets, defects, sectors, and intertwiners, equipped with the fusion of defects and fusion of sectors, forms a symmetric monoidal 3-category. This 3-category encodes the algebraic structure of the possible interactions among conformal field theories.
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Conformal nets V: dualizability
We prove that finite-index conformal nets are fully dualizable objects in the 3-category of conformal nets. Therefore, assuming the cobordism hypothesis applies, there exists a local framed topological field theory whose value on the point is any finite-index conformal net. Along the way, we prove a Peter-Weyl theorem for defects between conformal nets, namely that the annular sector of a finite defect is the sum of every sector tensor its dual.
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The Farrell-Jones Conjecture for mapping class groups
We prove the Farrell-Jones Conjecture for mapping class groups. The proof uses the Masur-Minsky theory of the large scale geometry of mapping class groups and the geometry of the thick part of Teichmueller space. The proof is presented in an axiomatic setup, extending the projection axioms of Bestvina-Bromberg-Fujiwara. More specifically, we prove that the action of the mapping class group on the Thurston compactification of Teichmueller space is finitely F-amenable for the family F consisting of virtual point stabilizers.
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K-theory and actions on Euclidean retracts
This note surveys axiomatic results for the Farrell-Jones Conjecture in terms of actions on Euclidean retracts and applications of these to GL_n(Z), relative hyperbolic groups and mapping class groups.
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Conformal nets II: conformal blocks
Conformal nets provide a mathematical formalism for conformal field theory. Associated to a conformal net with finite index, we give a construction of the `bundle of conformal blocks', a representation of the mapping class groupoid of closed topological surfaces into the category of finite-dimensional projective Hilbert spaces. We also construct infinite-dimensional spaces of conformal blocks for topological surfaces with smooth boundary. We prove that the conformal blocks satisfy a factorization formula for gluing surfaces along circles, and an analogous formula for gluing surfaces along intervals. We use this interval factorization property to give a new proof of the modularity of the category of representations of a conformal net.
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Conformal nets I: coordinate-free nets
We describe a coordinate-free perspective on conformal nets, as functors from intervals to von Neumann algebras. We discuss an operation of fusion of intervals and observe that a conformal net takes a fused interval to the fiber product of von Neumann algebras. Though coordinate-free nets do not a priori have vacuum sectors, we show that there is a vacuum sector canonically associated to any circle equipped with a conformal structure. This is the first in a series of papers constructing a 3-category of conformal nets, defects, sectors, and intertwiners.
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On the K-theory of groups with finite asymptotic dimension
It is proved that the assembly maps in algebraic K- and L-theory with respect to the family of finite subgroups is injective for groups with finite asymptotic dimension that admit a finite model for the classifying space for proper actions. The result also applies to certain groups that admit only a finite dimensional model for this space. In particular, it applies to discrete subgroups of virtually connected Lie groups.
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Coarse flow spaces for relatively hyperbolic groups
We introduce a coarse flow space for relatively hyperbolic groups and use it to verify a regularity condition for the action of relatively hyperbolic groups on their boundaries. As an application the Farrell-Jones Conjecture for relatively hyperbolic groups can be reduced to the peripheral subgroups (up to index 2 overgroups in the L-theory case).
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On proofs of the Farrell-Jones Conjecture
These notes contain an introduction to proofs of Farrell-Jones Conjecture for some groups and are based on talks given in Ohio, Oxford, Berlin, Shanghai, Münster and Oberwolfach in 2011 and 2012.
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Dualizability and index of subfactors
In this paper, we develop the theory of bimodules over von Neumann algebras, with an emphasis on categorical aspects. We clarify the relationship between dualizability and finite index. We also show that, for von Neumann algebras with finite-dimensional centers, the Haagerup L^2-space and Connes fusion are functorial with respect to homorphisms of finite index. Along the way, we describe a string diagram notation for maps between bimodules that are not necessarily bilinear.
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