Piotr Nowak's Homepage

Piotr Nowak

I was going to reference just a couple of Piotr's articles but then I thought that I might as well point to his entire home page. Everything is good here! In particular he shows that coarse embeddability into a Hilbert space (or into ell-one) is not the same as property A. The example is devastatingly simple: take the disjoint union of n-fold products of copies of some finite group (e.g. the group of order 2). Notice that the spaces here are quasi-isometric to cubes in R^n with the ell-one metric. If one took the ell-two (Euclidean) metric instead, wouldn't one get Yu's old counterexample to coarse Baum-Connes? Something interesting seems to be going on here.

[Of course these aren't bg spaces. Is there a bg space with the same property?]

[math/0511098] K-Theory of pseudodifferential operators with semi-periodic symbols

[math/0511098] K-Theory of pseudodifferential operators with semi-periodic symbols

Authors: Severino T. Melo, Cintia C. Silva
Subj-class: Operator Algebras; K-Theory and Homology
MSC-class: 46L80; 47G30

Let A denote the C*algebra of bounded operators on L2(R) generated by: (i) all multiplications a(M) by functions a\in C[-\infty,+\infty], (ii) all multiplications by 2\pi-periodic continuous functions and (iii) all Fourier multipliers F^{-1}b(M)F, where F denotes the Fourier transform and b is in C[-\infty,+\infty]. The Fredholm property for operators in A is governed by two symbols, the principal symbol \sigma and an operator-valued symbol \gamma. We give two proofs of the fact that K0(A) and K1(A) are isomorphic, respectively, to Z and Z\oplus Z: by computing the connecting mappings in the standard K-theory six-term exact sequences associated to \sigma and to \gamma. For the second computation, we prove that the image of \gamma is isomorphic to the direct sum of two copies of the crossed product of C[-\infty,+\infty] by an automorphism, and then use the Pimsner-Voiculescu exact sequence to compute its K-theory.

Another Coarse Geometry correction

See this link for another correction to LCG, this time in the statement and proof of Rosenblatt's theorem in chapter 3. Thanks to Steve Ferrt and his students for spotting this one.

Best wishes

John

[math/0509526] An analogue of the Novikov Conjecture in complex algebraic geometry

[math/0509526] An analogue of the Novikov Conjecture in complex algebraic geometry

We're familiar with the idea that statements about positive scalar curvature metrics (Gromov-Lawson-Rosenberg conjecture) and statements about higher signatures (Novikov conjecture) have certain parallels - ultimately they involve the higher index theorem applied to different elliptic operators, Dirac in the first case and signature in the second.

In this new paper Jonathan Rosenberg proposes a further family of statements involving higher index theory for the Dolbeault operator. These are statements in complex algebraic geometry about "higher Todd genera" for varieties.

This could be a whole new playground for higher index theorists.

Discrete Morse theory and graph braid groups

AGT 5 (2005) Paper 44 (Abstract)

Discrete Morse theory and graph braid groups
Daniel Farley, Lucas Sabalka
Abstract. If Gamma is any finite graph, then the unlabelled configuration space of n points on Gamma, denoted UC^n(Gamma), is the space of n-element subsets of Gamma. The braid group of Gamma on n strands is the fundamental group of UC^n(Gamma). We apply a discrete version of Morse theory to these UC^n(Gamma), for any n and any Gamma, and provide a clear description of the critical cells in every case. As a result, we can calculate a presentation for the braid group of any tree, for any number of strands. We also give a simple proof of a theorem due to Ghrist: the space UC^n(Gamma) strong deformation retracts onto a CW complex of dimension at most k, where k is the number of vertices in Gamma of degree at least 3 (and k is thus independent of n).

Subelliptic spinc Dirac operators

Charlie Epstein kindly gave me permission to publish this paper on index theory for generalizations of the d-bar Neumann problem and its relationship to index theory for Fourier integral operators.

[math/0505622] Free construction of CAT(1) spaces

[math/0505622] Free construction of CAT(1) spaces

[math/0508135] On the generalized Nielsen realization problem

[math/0508135] On the generalized Nielsen realization problem

Authors: Jonathan Block, Shmuel Weinberger
Subj-class: Geometric Topology
MSC-class: 57N

The main goal of this paper is to give the first examples of equivariant aspherical Poincare complexes, that are not realized by group actions on closed aspherical manifolds $M$. These will also provide new counterexamples to the Nielsen realization problem about lifting homotopy actions of finite groups to honest group actions. Our examples show that one cannot guarantee that a given action of a finitely generated group $\pi$ on Euclidean space extends to an action of $\Pi$, a group containing $\pi$ as a subgroup of finite index, even when all the torsion of $\Pi$ lives in $\pi$.

GT Vol 9 (2005) Paper 34 Hadamard spaces with isolated flats

GT Vol 9 (2005) Paper 34 (Abstract)

By Hruska and Kleiner with Hindawi

[math/0507542] A new kind of index theorem

[math/0507542] A new kind of index theorem

This is Ron Douglas' talk at the Kaminker retirement conference.

[hep-th/0507206] Dixmier traces on noncompact isospectral deformations

[hep-th/0507206] Dixmier traces on noncompact isospectral deformations

Ruan's lectures on orbifolds

Ruan's lectures at MSRI

[math/0502388] Quotients of Standard Hilbert Modules

[math/0502388] Quotients of Standard Hilbert Modules

Kind of a synthesis of BDF and algebraic geometry. By Bill Arveson.

A Comparison of Leibniz and Cyclic Homologies

Authors: Jerry Lodder
Comments: 14 pages
Subj-class: K-Theory and Homology; Algebraic Topology
MSC-class: 19D55; 17B66; 17A32

We relate Leibniz homology to cyclic homology by studying a map from a long exact sequence in the Leibniz theory to the ISB periodicity sequence in the cyclic theory. This provides a setting by which the two theories can be compared via the 5-lemma. We then show that the Godbillon-Vey invariant, as detected by the Leibniz homology of formal vector fields, maps to the Godbillon-Vey invariant as detected by the cyclic homology of the universal enveloping algebra of these vector fields.

Geometry of K\"ahler Metrics and Foliations by Holomorphic Discs

I've always been interested in holomorphic foliations...

[math/0506547] Coarse dimensions and partitions of unity

[math/0506547] Coarse dimensions and partitions of unity

More development of "coarse dimension theory" by professional dimension-theorists.

a space that can't embed in any uniformly convex Banach space

http://arxiv.org/abs/math/0506178

[math/0506544] Isometries, rigidity, and universal covers

[math/0506544] Isometries, rigidity, and universal covers

Farb and Weinberger classify aspherical Riemannian manifolds whose universal cover has "extra symmetry"

Coarse Homotopy - Trung's Thesis

This is a small advertising memo about part of Viet-Trung Luu's forthcoming thesis, which contains a very elegant unified treatment of various theorems about the 'coarse homotopy' or 'Lipschitz homotopy' invariance of the K-theory of C*(X). It formalizes the basic idea of the paper by Nigel and me in the Transactions, found here. One needs "C*(X) with coefficients", defined using Hilbert modules. Trung's approach has a geometric and an analytic part. The geometric part is to show that a coarse homotopy gives an element of C*(X; C[0,1]). The analytic part is to show that there is a natural pairing between the K-theory of C*(X;D) and the K-homology of D, with values in the K-theory of C*(X). Now one uses the known homotopy invariance of K-homology - the fact that the inclusions of 0 and 1 into [0,1] induce isomorphisms on K-homology - to conclude the coarse homotopy invariance of K_*(C*(X)).

[math/0506361] Property (T) and rigidity for actions on Banach spaces

[math/0506361] Property (T) and rigidity for actions on Banach spaces

Here the authors study variations of property T for actions on Banach spaces, especially Lp. When p is not 2 the equivalence between the original version of property T (isolation of the trivial representation) and the fixed point version F (affine isometric actions) does not hold.
They prove that every property T group has T(L^p) for all p, and F(L^p) for p<2+\epsilon.