296 research outputs found
On a class of extremal solutions of a moment problem for rational matrix-valued functions in the nondegenerate case II
AbstractThe main theme of this paper is the discussion of a family of extremal solutions of a finite moment problem for rational matrix functions in the nondegenerate case. We will point out that each member of this family is extremal in several directions. Thereby, the investigations below continue the studies in Fritzsche et al. (in press) [1]. In doing so, an application of the theory of orthogonal rational matrix functions with respect to a nonnegative Hermitian matrix Borel measure on the unit circle is used to get some insights into the structure of the extremal solutions in question. In particular, we explain characterizations of these solutions in the whole solution set in terms of orthogonal rational matrix functions. We will also show that the associated Riesz–Herglotz transform of such a particular solution admits specific representations, where orthogonal rational matrix functions are involved
On an Interpolation Problem for J-Potapov Functions
Let, J, be an m-by-m-signature matrix and let D be the open unit disk in the
complex plane. Denote by P{J,0}(D) the class of all meromorphic
m-by-m-matrix-valued functions, f, in D which are holomorphic at 0 and take
J-contractive values at all points of D at which f is holomorphic. The central
theme of this paper is the study of the following interpolation problem:
Let n be a nonnegative integer, and let A_0, A_1, ..., A_n be a sequence of
complex m-by-m-matrices. Describe the set of all matrix-valued functions, f,
belonging to the class P{J,0}(D), such that the first n+1 Taylor coefficients
of f coincide with A_0, A_1, ..., A_n. In particular, we characterize the case
that this set is non-empty.
In this paper, we will solve this problem in the most general case. Moreover,
in the non-degenerate case we will give a description of the corresponding Weyl
matrix balls. Furthermore, we will investigate the limit behaviour of the Weyl
matrix balls associated with the functions belonging to some particular
subclass of P{J,0}(D).Comment: 44 page
The truncated Hamburger matrix moment problems in the nondegenerate and degenerate cases, and matrix continued fractions
AbstractThe present paper deals simultaneously with the nondegenerate and degenerate truncated Hamburger matrix moment problems in a unified way based on the use of the Schur algorithm involving matrix continued fractions. A full analysis of them together with a relative matrix moment problem on the real axis is given. With the help of the correspondence between the moment problem on the real axis and the Nevanlinna-Pick (NP) interpolation, the solutions of the nontangential NP interpolation in the Nevanlinna class are derived as an application
Quantum Control Landscapes
Numerous lines of experimental, numerical and analytical evidence indicate
that it is surprisingly easy to locate optimal controls steering quantum
dynamical systems to desired objectives. This has enabled the control of
complex quantum systems despite the expense of solving the Schrodinger equation
in simulations and the complicating effects of environmental decoherence in the
laboratory. Recent work indicates that this simplicity originates in universal
properties of the solution sets to quantum control problems that are
fundamentally different from their classical counterparts. Here, we review
studies that aim to systematically characterize these properties, enabling the
classification of quantum control mechanisms and the design of globally
efficient quantum control algorithms.Comment: 45 pages, 15 figures; International Reviews in Physical Chemistry,
Vol. 26, Iss. 4, pp. 671-735 (2007
Rigidity and quasi-rigidity of extremal cycles in Hermitian symmetric spaces
I use local differential geometric techniques to prove that the algebraic
cycles in certain extremal homology classes in Hermitian symmetric spaces are
either rigid (i.e., deformable only by ambient motions) or quasi-rigid (roughly
speaking, foliated by rigid subvarieties in a nontrivial way).
These rigidity results have a number of applications: First, they prove that
many subvarieties in Grassmannians and other Hermitian symmetric spaces cannot
be smoothed (i.e., are not homologous to a smooth subvariety). Second, they
provide characterizations of holomorphic bundles over compact Kahler manifolds
that are generated by their global sections but that have certain polynomials
in their Chern classes vanish (for example, c_2 = 0, c_1c_2 - c_3 = 0, c_3 = 0,
etc.).Comment: 113 pages, 6 figures, latex2e with packages hyperref, amsart,
graphicx. For Version 2: Many typos corrected, important references added
(esp. to Maria Walters' thesis), several proofs or statements improved and/or
correcte
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