Bøger i Operator Theory: Advances and Applications serien

This volume contains the proceedings of the international conference on " Operator Calculus and Spectral Theory" in Lambrecht, Germany, Decem­ ber 9. - 14. , 1991, sponsored by the Deutsche Forschungsgemeinschaft, and by the Karl-Weierstrass-Institute of Mathematics, Berlin. The idea was to bring together specialists from different areas of modern analysis, geometry and mathematical physics, in a similar spirit as in the earlier series of conferences of the Karl-Weierstrass- Institute (Ludwigsfelde, 1976; Reinhardsbrunn, 1985; Holzhau, 1988; Breitenbrunn,1990). Berlin, Mainz M. Demuth B. Gramsch B. -W. Schulze List of talks given in La. brecht. 8. - 14. December 91 Name Title S. Albeverio Some recent deveiopments in Dirichlet forms and associated processes U. Bunke On the spectral flow of Dirac operators with negative definite functions as symbols - applications in probability theory and mathematical physics M. Combescure Recurrent versus diffusive behaviour for time-dependent quantum Hamiltonian E. B. Davies Analysis on graphs and noncommutative geometry M. Demuth On stochastic spectral analysis and asymptotics for Schrodinger operators P. Duclos A global approach to the location of quantum resonances v. v. Egorov On negative spectrum of elliptic operators V. EnB Non-threshold states of long-range N-body problems B. Gramsch -algebras and microlocal analysis P. B. Gilkey On the index of geometrical operators for Riemannian manifolds with boundary B. Helffer Spectral problems in statistical mechanics R. Hempe I Second order pertubations of elliptic operators with periodic principal part T. Ichinose On the Weyl quantized relativistic Hamiltonian V. V.

This volume, addressed to researchers and postgraduate students, compiles up-to-date research and expository papers on different aspects of complex analysis, including relations to operator theory and hypercomplex analysis. Subjects include the Schroedinger equation, subelliptic operators, Lie algebras and superalgebras, among others.

Theorems of factorising matrix functions and the operator identity method play an essential role in this book in constructing the spectral theory (direct and inverse problems) of canonical differential systems. Includes many varied applications of the general theory.

th This volume is dedicated to Carl Pearcy on his 60 birthday. It collects recent contributions to operator theory, nonselfadjoint operator algebras, measure the­ ory, and the theory of moments by several of the lead­ ing specialists in those areas. Many of the contributors are collaborators or former students of Carl Pearcy, and the variety of the topics bears witness to the wide range of his work and interests. The editors were helped by many in the compi­ lation of this volume. Srdjan Petrovic helped com­ pile Carl's list of publications, while Arlen Brown and George Exner helped in writing the biographical and mathematical sketch. The work of many referees, who must remain anonymous, was very valuable. Israel Gohberg suggested that we publish this volume in the distinguished series Operator Theory: Advances and Applications. The whole volume was expertly typeset by Elena Fraboschi. We wish to extend to all of these people our heartfelt thanks. CARL M. PEARCY Carl M. Pearcy: A Biographical Sketch H. BERCOVICI fj C. FOIAS Carl Mark Pearcy, Jr. was born on August 23, 1935 in Beaumont, Texas. He was the eldest of two sons of Carl Mark Pearcy, Sr., and Carrie Edith (Tilbury) Pearcy.

This book presents a unified approach for solving both stationary and nonstationary interpolation problems, in finite or infinite dimensions, based on the commutant lifting theorem from operator theory and the state space method from mathematical system theory.

Pseudo-differential operators belong to the most powerful tools in the analysis of partial differential equations. Basic achievements in the early sixties have initiated a completely new understanding of many old and important problems in analy­ sis and mathematical physics. The standard calculus of pseudo-differential and Fourier integral operators may today be considered as classical. The development has been continuous since the early days of the first essential applications to ellip­ ticity, index theory, parametrices and propagation of singularities for non-elliptic operators, boundary-value problems, and spectral theory. The basic ideas of the calculus go back to Giraud, Calderon, Zygmund, Mikhlin, Agranovich, Dynin, Vishik, Eskin, and Maslov. Subsequent progress was greatly stimulated by the classical works of Kohn, Nirenberg and Hormander. In recent years there developed a new vital interest in the ideas of micro­ local analysis in connection with analogous fields of applications over spaces with singularities, e.g. conical points, edges, corners, and higher singularities. The index theory for manifolds with singularities became an enormous challenge for analysists to invent an adequate concept of ellipticity, based on corresponding symbolic structures. Note that index theory was another source of ideas for the later development of the theory of pseudo-differential operators. Let us mention, in particular, the fundamental contributions by Gelfand, Atiyah, Singer, and Bott.

Generalized Schur functions are scalar- or operator-valued holomorphic functions such that certain associated kernels have a finite number of negative squares.

The following tract is divided into three parts: Hilbert spaces and their (bounded and unbounded) self-adjoint operators, linear Hamiltonian systemsand their scalar counterparts and their application to orthogonal polynomials.

PREFACE The classic statement of the Riemann boundary problem consists in finding a function (z) which is analytic and bounded in two domains D+ and D-, with a common boundary - a smooth closed contour L admitting a continuous extension onto L both from D+ and D- and satisfying on L the boundary condition +(t) = G(t)-(t) + g(t).

A collection of papers on different aspects of operator theory and complex analysis, covering the recent achievements of the Odessa-Kharkov school, where Potapov was very active. The book appeals to a wide group of mathematicians and engineers, and much of the material can be used for advanced courses and seminars.

Kaashoek, "Bitangential interpola tion for input-output operators of time varying systems: the discrete time case" contains solutions of time varying interpolation problems.

Since 1976 the Institute of Mathematics of the Romanian Academy (formerly the Department of Mathematics of INCREST) and the Faculty of Mathematics (formerly the Faculty of Sciences) of the University ofTimi~oara have organized several Con ferences on Operator Theory.

This volume is dedicated to Tsuyoshi Ando, a foremost expert in operator theory, matrix theory, complex analysis, and their applications, on the occasion of his 60th birthday. This volume also contains papers on interpolation and completion problems, factorization problems and problems connected with complex analysis.

This monograph is the second volume of a graduate text book on the modern theory of linear one-dimensional singular integral equations. The authors Ramat-Aviv, Ramat-Gan, May 26, 1991 11 Introduction This book is the second volume of an introduction to the theory of linear one-dimensional singular integral operators.

The contributions in this volume are dedicated to Vladimir G. Maz'ya and are par­ tially based on talks given at the conference "Functional Analysis, Partial Differ­ ential Equations, and Applications", which took place at the University of Rostock from August 31 to September 4, 1998, to honour Prof. Maz'ya. This conference (a satellite meeting of the ICM) gave an opportunity to many friends and colleagues from all over the world to honour him. This academic community is very large. The scientific field of Prof. Maz'ya is impressively broad, which is reflected in the variety of contributions included in the volumes. Vladimir Maz'ya is the author and co-author of many publications (see the list of publications at the end of this volume), the topics of which extend from functional analysis, function theory and numerical analysis to partial differential equations and their broad applications. Vladimir G. Maz'ya provided significant contributions, among others to the the­ ory of Sobolev spaces, the capacity theory, boundary integral methods, qualitative and asymptotic methods of analysis of linear and nonlinear elliptic differential equations, the Cauchy problem for elliptic and hyperbolic equations, the theory of multipliers in spaces of differentiable functions, maximum principles for elliptic and parabolic systems, and boundary value problems in domains with piecewise smooth boundaries. Surveys on Maz'ya's work in different fields of mathematics and areas, where he made essential contributions, form a major part of the present first volume of The Maz'ya Anniversary Collection.

A E r* J J J J J where Aj is a square matrix of size nj x n* say. B and C are j j j matrices of sizes n. J x J J and Aj = Aj - BjC have no eigenvalues on r . 1) the functions j j Wj are normalized to I at infinity.

This book aims to present the theory of interpolation for rational matrix functions as a recently matured independent mathematical subject with its own problems, methods and applications.

Classical H~ interpolation theory was conceived at the beginning of the century by C. Caratheodory, L. Fejer and I. Schur. The basic method, due to Schur, in solving these problems consists in applying the Möbius transform to peel off the data. In 1967, D. Sarason encompassed these classical interpolation problems in a representation theorem of operators commuting with special contractions. Shortly after that, in 1968, B. Sz.­ Nagy and C. Foias obtained a purely geometrical extension of Sarason's results. Actually, their result states that operators intertwining restrictions of co-isometries can be extended, by preserving their norm, to operators intertwining these co-isometries; starring with R. G. Douglas, P. S. Muhly and C. Pearcy, this is referred to as the commutant lifting theorem. In 1957, Z. Nehari considered an L ~ interpolation problern which in turn encompassed the same classical interpolation problems, as well as the computation of the distance of a function f in L ~ to H~. At about the sametime as Sarason's work, V. M.

The notions of positive functions and of reproducing kernel Hilbert spaces play an important role in various fields of mathematics, such as stochastic processes, linear systems theory, operator theory, and the theory of analytic functions.

In September 1998, during the 'International Workshop on Analysis and Vibrat ing Systems' held in Canmore, Alberta, Canada, it was decided by a group of participants to honour Peter Lancaster on the occasion of his 70th birthday with a volume in the series 'Operator Theory: Advances and Applications'.

This volume contains the proceedings of "PDE 2000", the international conference on partial differential equations held July 24 -28, 2000, in Clausthal. The confer­ ence took place during the EXPO 2000 and was sponsored by the Land Nieder­ sachsen, the Deutsche Forschungsgemeinschaft, the Bergstadt Clausthal-Zellerfeld and the Kreissparkasse Clausthal-Zellerfeld. This conference continues a series: Ludwigfelde 1976, Reinhardsbrunn 1985, Holz­ hau 1988, Breitenbrunn 1990, Lambrecht 1991 (proceedings in Operator Theory: Advances and Applications, Vol. 57, Birkhauser Verlag 1992), Potsdam 1992 and 1993, Holzhau 1994 (proceedings in Operator Theory: Advances and Applications, Vol. 78, Birkhauser Verlag 1995), Caputh 1995 and Potsdam 1996 (proceedings in Mathematical Research, Vol. 100, Akademie Verlag 1997). The intention of the organizers was to bring together specialists from different areas of modern analysis, mathematical physics and geometry, to discuss not only the recent progress in their own fields but also the interaction between these fields. The special topics of the conference were spectral and scattering theory, semiclas­ sical and asymptotic analysis, pseudodifferential operators and their relation to geometry, as well as partial differential operators and their connection to stochas­ tic analysis and to the theory of semigroups. The scientific advisory board of the conference in Clausthal consisted of M. Ben­ Artzi (Jerusalem), Chen Hua (Peking), M. Demuth (Clausthal), T. Ichinose (Kanazawa), 1. Rodino (Thrin), B.-W. Schulze (Potsdam) and J. Sjostrand (Paris).

This book is de­ voted to some topical prob­ lems and various applica­ tions of Operator Theory and to its interplay with many other fields of analysis as modern approximation the­ ory, theory of dynamic sys­ tems, harmonic analysis and complex analysis. It consists of 20 carefully selected sur­ veys and research-expository papers. Their scope gives a representative status report on the field drawing a pic­ ture of a rapidly developing domain of analysis. An abun­ dance of references completes the picture. All papers included in the volume originate from lectures delivered at the l1th edition of the International Workshop on Operator The­ ory and its Applications (IWOTA-2000, June 13-16, Bordeaux). Some information about the conference, including the complete list of participants, can be found on forthcoming pages. The editors are indebted to A.Sudakov for helping them in polishing and assembling original TeX files. A. Borichev and N. Nikolski Talence, May 2001 v vii International Workshop on Operator Theory and Its Applications (June 13-June 16, 2000, Universite Bordeaux 1) The International Workshop on Operator Theory and its Applications (IWOTA) is a satellite meeting of the international symposium on the Mathe­ matical Theory of Networks and Systems (MNTS). In 2000, the MNTS is held in Perpignan, France, June 19-23. IWOTA 2000 was the eleventh workshop of this kind.

This book is devoted to new classes of parabolic differential and pseudo-differential equations extensively studied in the last decades, such as parabolic systems of a quasi-homogeneous structure, degenerate equations of the Kolmogorov type, pseudo-differential parabolic equations, and fractional diffusion equations.

Our goal is to find Grabner bases for polynomials in four different sets of expressions: 1 x- , (1 - x)-1 (RESOL) X, 1 x- (1 - xy)-1 (EB) X, , y-1, (1-yx)-1 y, (1_y)-1 (1-x)-1 (preNF) (EB) plus and (1 - xy)1/2 (1 - yx )1/2 (NF) (preNF) plus and Most formulas in the theory of the Nagy-Foias operator model [NF] are polynomials in these expressions where x = T and y = T*. Complicated polynomials can often be simplified by applying "replacement rules". For example, the polynomial (1 - xy)-2 - 2xy(1-xy)-2 + xy2 (1 - xy)-2 -1 simplifies to O. This can be seen by three applications of the replacement rule (1-xy) -1 xy -t (1 - xy)-1 -1 which is true because of the definition of (1-xy)-1. A replacement rule consists of a left hand side (LHS) and a right hand side (RHS). The LHS will always be a monomial. The RHS will be a polynomial whose terms are "simpler" (in a sense to be made precise) than the LHS. An expression is reduced by repeatedly replacing any occurrence of a LHS by the corresponding RHS. The monomials will be well-ordered, so the reduction procedure will terminate after finitely many steps. Our aim is to provide a list of substitution rules for the classes of expressions above. These rules, when implemented on a computer, provide an efficient automatic simplification process. We discuss and define the ordering on monomials later.

Linear differential equations with periodic coefficients constitute a well developed part of the theory of ordinary differential equations [17, 94, 156, 177, 178, 272, 389].

Since the introduction of the functional classes HW (lI) and WT HW (lI) and their peri­ odic analogs Hw (1I') and ~ (1I'), defined by a concave majorant w of functions and their rth derivatives, many researchers have contributed to the area of ex­ tremal problems and approximation of these classes by algebraic or trigonometric polynomials, splines and other finite dimensional subspaces. In many extremal problems in the Sobolev class W~ (lI) and its periodic ana­ log W~ (1I') an exceptional role belongs to the polynomial perfect splines of degree r, i.e. the functions whose rth derivative takes on the values -1 and 1 on the neighbor­ ing intervals. For example, these functions turn out to be extremal in such problems of approximation theory as the best approximation of classes W~ (lI) and W~ (1I') by finite-dimensional subspaces and the problem of sharp Kolmogorov inequalities for intermediate derivatives of functions from W~. Therefore, no advance in the T exact and complete solution of problems in the nonperiodic classes W HW could be expected without finding analogs of polynomial perfect splines in WT HW .

It contains a selection of original research papers which cover a broad spectrum of topics about operators and semigroups of operators on Banach lattices, analysis in function spaces and integration theory.

The fates of important mathematical ideas are varied. Polishchuk, reconstructs and develops one of the dir.ctions in functional analysis that originated from Hadamard and Gateaux and was newly thought over and taken as the basis of a prospective theory by Paul Levy.