3,726 research outputs found
Identification of stochastic operators
Based on the here developed functional analytic machinery we extend the
theory of operator sampling and identification to apply to operators with
stochastic spreading functions. We prove that identification with a delta train
signal is possible for a large class of stochastic operators that have the
property that the autocorrelation of the spreading function is supported on a
set of 4D volume less than one and this support set does not have a defective
structure. In fact, unlike in the case of deterministic operator
identification, the geometry of the support set has a significant impact on the
identifiability of the considered operator class. Also, we prove that,
analogous to the deterministic case, the restriction of the 4D volume of a
support set to be less or equal to one is necessary for identifiability of a
stochastic operator class
emgr - The Empirical Gramian Framework
System Gramian matrices are a well-known encoding for properties of
input-output systems such as controllability, observability or minimality.
These so-called system Gramians were developed in linear system theory for
applications such as model order reduction of control systems. Empirical
Gramian are an extension to the system Gramians for parametric and nonlinear
systems as well as a data-driven method of computation. The empirical Gramian
framework - emgr - implements the empirical Gramians in a uniform and
configurable manner, with applications such as Gramian-based (nonlinear) model
reduction, decentralized control, sensitivity analysis, parameter
identification and combined state and parameter reduction
Local and Global Well-Posedness for Aggregation Equations and Patlak-Keller-Segel Models with Degenerate Diffusion
Recently, there has been a wide interest in the study of aggregation
equations and Patlak-Keller-Segel (PKS) models for chemotaxis with degenerate
diffusion. The focus of this paper is the unification and generalization of the
well-posedness theory of these models. We prove local well-posedness on bounded
domains for dimensions and in all of space for , the
uniqueness being a result previously not known for PKS with degenerate
diffusion. We generalize the notion of criticality for PKS and show that
subcritical problems are globally well-posed. For a fairly general class of
problems, we prove the existence of a critical mass which sharply divides the
possibility of finite time blow up and global existence. Moreover, we compute
the critical mass for fully general problems and show that solutions with
smaller mass exists globally. For a class of supercritical problems we prove
finite time blow up is possible for initial data of arbitrary mass.Comment: 31 page
J-spectral factorization and equalizing vectors
For the Wiener class of matrix-valued functions we provide necessary and sufficient conditions for the existence of a -spectral factorization. One of these conditions is in terms of equalizing vectors. A second one states that the existence of a -spectral factorization is equivalent to the invertibility of the Toeplitz operator associated to the matrix to be factorized. Our proofs are simple and only use standard results of general factorization theory. Note that we do not use a state space representation of the system. However, we make the connection with the known results for the Pritchard-Salamon class of systems where an equivalent condition with the solvability of an algebraic Riccati equation is given. For Riesz-spectral systems another necessary and sufficient conditions for the existence of a -spectral factorization in terms of the Hamiltonian is added
Scarred eigenstates for arithmetic toral point scatterers
We investigate eigenfunctions of the Laplacian perturbed by a delta potential
on the standard tori in dimensions .
Despite quantum ergodicity holding for the set of "new" eigenfunctions we show
that there is scarring in the momentum representation for , as well as
in the position representation for (i.e., the eigenfunctions fail to
equidistribute in phase space along an infinite subsequence of new
eigenvalues.) For , scarred eigenstates are quite rare, but for
scarring in the momentum representation is very common --- with denoting the counting function for the new eigenvalues below
, there are eigenvalues corresponding to momentum
scarred eigenfunctions.Comment: 31 pages, 1 figur
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