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Our main result is a theorem saying that a bounded operator $A$ on a Hilbert space belongs to a certain set associated with its self-commutator $[A^*,A]$, provided that $A-zI$ can be approximated by invertible operators for all complex numbers $z$. The theorem remains valid in a general $C^*$-algebra of real rank zero under the assumption that $A-zI$ belong to the closure of the connected component of unity in the set of invertible elements. This result implies the Brown--Douglas--Fillmore theorem and Huaxin Lin's theorem on almost commuting matrices. Moreover, it allows us to refine the former and to extend the latter to operators of infinite rank and other norms (including the Schatten norms on the space of matrices). The proof is based on an abstract theorem, which states that a normal element of a $C^*$-algebra of real rank zero satisfying the above condition has a resolution of the identity associated with any open cover of its spectrum., Version 2: results related to the BDF theorem are significantly improved, few references and remarks are added, and introduction is modified to reflect these changes. 27 pages

The paper extends Birkhoff's theorem on doubly stochastic matrices to some countable families of discrete probability spaces with nonempty intersections. We join every two elements lying in the same probability space by an edge and formulate our results in terms of the obtained graph.

Let F be a real-valued convex function on the complex plane, and let Ps(b) be a pseudodifferential operator with symbol b. Under certain natural assumptions about properties of pseudodifferential operators, we prove that the sum of F(z) over all eigenvalues z of the operator Ps(b) counted with their multiplicities does not exceed the real part of the trace of Ps(F(b)) modulo a term of the same order as the error term in the Garding inequality., Comment: 9pp

We study asymptotic distribution of eigenvalues of the Laplacian on a bounded domain in ℝ n . Our main results include an explicit remainder estimate in the Weyl formula for the Dirichlet Laplacian on an arbitrary bounded domain, sufficient conditions for the validity of the Weyl formula for the Neumann Laplacian on a domain with continuous boundary in terms of smoothness of the boundary and a remainder estimate in this formula. In particular, we show that the Weyl formula holds true for the Neumann Laplacian on a Lip α -domain whenever (d−1)/α

We obtain an order sharp estimate for the distance from a given bounded operator $A$ on a Hilbert space to the set of normal operators in terms of $\|[A,A^*]\|$ and the distance to the set of invertible operators. A slightly modified estimate holds in a general $C^*$-algebra of real rank zero., Comment: 22 pages, 10 figures. The introduction has been rewritten as the referees suggested. Some other minor amendments in the text were made, J. Amer. Math. Soc. electronically published on January 8, 2015

The main aim of the paper is to present a general version of the Fourier Tauberian theorem for monotone functions. This result, together with Berezin's inequality, allows us to obtain a refined version the Li-Yau estimate for the counting function of the Dirichlet Laplacian which implies the Li-Yau estimate itself and, at the same time, the asymptotic results obtained by the variational method., Comment: 15 pages

We analyze the semiclassical limit of spectral theory on manifolds whose metrics have jump-like discontinuities. Such systems are quite different from manifolds with smooth Riemannian metrics because the semiclassical limit does not relate to a classical flow but rather to branching (ray-splitting) billiard dynamics. In order to describe this system we introduce a dynamical system on the space of functions on phase space. To identify the quantum dynamics in the semiclassical limit we compute the principal symbols of the Fourier integral operators associated to reflected and refracted geodesic rays and identify the relation between classical and quantum dynamics. In particular we prove a quantum ergodicity theorem for discontinuous systems. In order to do this we introduce a new notion of ergodicity for the ray-splitting dynamics. The paper contains an Appendix written by Yves Colin de Verdiere in which a non-trivial class of examples is constructed., Comment: 51 pages, 6 figures, proof in Appendix A corrected and some more typos fixed, error in the formula for the transmission and reflection coefficient due to different choice of boundary conditions corrected, Appendix by Yves Colin de Verdiere

The paper presents a lower bound for the number of eigenvalues of an integral operator k with continuous kernel\( \mathcal{K} \) lying in the interval \( (-\infty,t){\rm with} \; t\leq 0 \). The estimate is given in terms of some integrals of \( \mathcal{K} \)

In this paper we develop a new approach to the theory of Fourier integral operators. It allows us to represent the Schwartz kernel of a Fourier integral operator by one oscillatory integral with a complex phase function. We consider Fourier integral operators associated with canonical transformations, having in mind applications to hyperbolic equations. As a by-product we obtain yet another formula for the Maslov index. (C) 1994 John Wiley & Sons, Inc.

The main aim of the paper is to demonstrate the advantage of a coordinate-free approach to the theory of pseudodifferential operators. We explain how one can define symbols and construct a symbolic calculus without using local coordinates, briefly review some known definitions and results, and discuss possible applications and further developments.

We present explicit estimates for the remainder in the Weyl formula for the Laplace operator on a domain Ω, which involve only the most basic characteristics of Ω and hold under minimal assumptions about the boundary \(\partial \Omega\).

We study the relation between the spectrum of a self-adjoint operator and its multidimensional numerical range. It turns out that the multidimensional numerical range is a convex set whose extreme points are sequences of eigenvalues of the operator. Every collection of eigenvalues which can be obtained by the Rayleigh--Ritz formula generates an extreme point of the multidimensional numerical range. However, it may also have other extreme points., 35 pages

This volume brings together lectures from an instructional meeting on spectral theory and geometry held under the auspices of the International Centre for Mathematical Sciences in Edinburgh. The contributions here come from world experts and many are much expanded versions of the lectures they gave. Together they survey the core material and go beyond to reach deeper results. For graduate students and experts alike, this book will be a highly useful resource.