arXiv subjects

Sascha Trostorff

Publications and source records attributed to Sascha Trostorff.

At least 37 records · Page 2Linked to original sources

arXiv2019

A Hilbert space approach to fractional difference equations

We formulate fractional difference equations of Riemann-Liouville and Caputo type in a functional analytical framework. Main results are existence of solutions on Hilbert space-valued weighted sequence spaces and a condition for stability of linear fractional difference equations. Using a functional calculus, we relate the fractional sum to fractional powers of the operator $1 - τ^{-1}$ with the right shift $τ^{-1}$ on weighted sequence spaces. Causality of the solution operator plays a crucial role for the description of initial value problems.

math.DS↗

arXiv2018

A Hilbert space approach to difference equations

We consider general difference equations $u_{n+1} = F(u)_n$ for $n \in \mathbb{Z}$ on exponentially weighted $\ell_2$ spaces of two-sided Hilbert space valued sequences $u$ and discuss initial value problems. As an application of the Hilbert space approach, we characterize exponential stability of linear equations and prove a stable manifold theorem for causal nonlinear difference equations.

math.DS↗

arXiv2018

On the Well-posedness of a Class of Non-Autonomous SPDEs: An Operator-Theoretical Perspective

We further elaborate on the solvability of stochastic partial differential equations (SPDEs). We shall discuss non-autonomous partial differential equations with an abstract realization of the stochastic integral on the right-hand side. Our approach allows the treatment of equations with mixed type, where classical solution strategies fail to work. The approach extends prior observations in [Süß, A. \& Waurick, M. A Solution Theory for a General Class of SPDEs. \emph{Stochastics and Partial Differential Equations: Analysis and Computations}, 2017, 5, 278-318], where the respective results were obtained for linear autonomous equations and (multiplicative) white noise.

math.AP↗

arXiv2017

On higher index differential-algebraic equations in infinite dimensions

We consider initial value problems for differential-algebraic equations in a possibly infinite-dimensional Hilbert space. Assuming a growth condition for the associated operator pencil, we prove existence and uniqueness of solutions for arbitrary initial values in a distributional sense. Moreover, we construct a nested sequence of subspaces for initial values in order to obtain classical solutions.

math.CA↗

arXiv2017

On Differential-Algebraic Equations in Infinite Dimensions

We consider a class of differential-algebraic equations (DAEs) with index zero in an infinite dimensional Hilbert space. We define a space of consistent initial values, which lead to classical continuously differential solutions for the associated DAE. Moreover, we show that for arbitrary initial values we obtain mild solutions for the associated problem. We discuss the asymptotic behaviour of solutions for both problems. In particular, we provide a characterisation for exponential stability and exponential dichotomies in terms of the spectrum of the associated operator pencil.

math.FA↗

arXiv2017

Exponential Stability and Initial Value Problems for Evolutionary Equations

In this thesis we consider so-called linear evolutionary problems, a class of linear partial differential equations covering classical elliptic, parabolic and hyperbolic equations from mathematical physics as well as classes of integro-differenital equations, fractional differential equations and delay equations. We address the well-posedness of the problems in a pure Hilbert space setting. Moreover, the exponential stability and the regularity of the problems are studied. In particular, a Hille-Yosida type result is proved, to obtain a strongly continuous semigroup on a suitable state space consisting of admissible initial values and pre-histories. The results are illustrated by various examples.

math.AP↗

arXiv2016

Numerical methods for changing type systems

In this note we develop a numerical method for partial differential equations with changing type. Our method is based on a unified solution theory found by Rainer Picard for several linear equations from mathematical physics. Parallel to the solution theory already developed, we frame our numerical method in a discontinuous Galerkin approach in space-time with certain exponentially weighted spaces.

math.NA↗

arXiv2016

On the weighted Gauß--Radau Quadrature

In this short note, we collect some facts on the weighted Gauß--Radau quadrature. In particular, we focus on the location of the Gauß--Radau points being a continuous function of the $L^1$-weighting function.

math.HO↗

arXiv2016

A remark on local activity and passivity

We study local activity and its opposite, local passivity, for linear systems and show that generically an eigenvalue of the system matrix with positive real part implies local activity. If all state variables are port variables we prove that the system is locally active if and only if the system matrix is not dissipative. Local activity was suggested by Leon Chua as an indicator for the emergence of complexity of nonlinear systems. We propose an abstract scheme which indicates how local activity could be applied to nonlinear systems and list open questions about possible consequences for complexity.

math.DS↗

arXiv2016

Well-posedness of Linear Integro-Differential Equations with Operator-valued Kernels

We study linear integro-differential equations in Hilbert spaces with operator-valued kernels and give sufficient conditions for the well-posedness. We show that several types of integro-differential equations are covered by the class of evolutionary equations introduced in [R. Picard. A structural observation for linear material laws in classical mathematical physics. Math. Methods Appl. Sci., 32(14):1768-1803, 2009]. We therefore give criteria for the well-posedness within this framework. As an example we apply our results to the equations of visco-elasticity.

math.AP↗

arXiv2016

On Maximal Regularity for a Class of Evolutionary Equations

The issue of so-called maximal regularity is discussed within a Hilbert space framework for a class of evolutionary equations. Viewing evolutionary equations as a sums of two unbounded operators, showing maximal regularity amounts to establishing that the operator sum considered with its natural domain is already closed. For this we use structural constraints of the coefficients rather than semi-group strategies or sesqui-linear form methods, which would be difficult to come by for our general problem class. Our approach, although limited to the Hilbert space case, complements known strategies for approaching maximal regularity and extends them in a different direction. The abstract findings are illustrated by re-considering some known maximal regularity results within the framework presented.

math.AP↗

arXiv2016

On Wellposedness for Some Thermo-Piezo-Electric Coupling Models

There is an increasing reliance on mathematical modelling to assist in the design of piezoelectric ultrasonic transducers since this provides a cost-effective and quick way to arrive at a first prototype. Given a desired operating envelope for the sensor the inverse problem of obtaining the associated design parameters within the model can be considered. It is therefore of practical interest to examine the well-posedness of such models. There is a need to extend the use of such sensors into high temperature environments and so this paper shows, for a broad class of models, the well-posedness of the magneto-electro-thermo-elastic problem. Due to its widespread use in the literature, we also show the well-posedness of the quasi-electrostatic case

math.AP↗

arXiv2015

A Note on a Two-Temperature Model in Linear Thermoelasticity

We discuss the so-called two-temperature model in linear thermoelasticity and provide a Hilbert space framework for proving well-posedness of the equations under consideration. With the abstract perspective of evolutionary equations, the two-temperature model turns out to be a coupled system of the elastic equations and an abstract ode. Following this line of reasoning, we propose another model being entirely an abstract ode. We highlight also an alternative way for a two-temperature model, which might be of independent interest.

math.AP↗

arXiv2015

Exponential stability for second order evolutionary problems

We study the exponential stability of evolutionary equations. The focus is laid on second order problems and we provide a way to rewrite them as a suitable first order evolutionary equation, for which the stability can be proved by using frequency domain methods. The problem class under consideration is broad enough to cover integro-differential equations, delay-equations and classical evolution equations within a unified framework.

math.AP↗

arXiv2015

On Abstract $\mathrm{grad}-\mathrm{div}$ Systems

For a large class of dynamical problems from mathematical physics the skew-selfadjointness of a spatial operator of the form $A=\left(\begin{array}{cc} 0 & -C^{*}\\ C & 0 \end{array}\right)$, where $C:D\left(C\right)\subseteq H_{0}\to H_{1}$ is a closed densely defined linear operator, is a typical property. Guided by the standard example, where $C=\mathrm{grad}=\left(\begin{array}{c} \partial_{1}\\ \vdots\\ \partial_{n} \end{array}\right)$ (and $-C^{*}=\mathrm{div}$, subject to suitable boundary constraints), an abstract class of operators $C=\left(\begin{array}{c} C_{1}\\ \vdots\\ C_{n} \end{array}\right)$ is introduced (hence the title). As a particular application we consider a non-standard coupling mechanism and the incorporation of diffusive boundary conditions both modeled by setting associated with a skew-selfadjoint spatial operator $A$.

math.AP↗

arXiv2015

On a class of block operator matrices in system theory

We consider a class of block operator matrices arising in the study of scattering passive systems, especially in the context of boundary control problems. We prove that these block operator matrices are indeed a subclass of block operator matrices considered in [Trostorff: A characterization of boundary conditions yielding maximal monotone operators. J. Funct. Anal., 267(8): 2787--2822, 2014], which can be characterized in terms of an associated boundary relation.

math.FA↗

arXiv2014

On Some Models in Linear Thermo-Elasticity with Rational Material Laws

We shall consider some common models in linear thermo-elasticity within a common structural framework. Due to the flexibility of the structural perspective we will obtain well-posedness results for a large class of generalized models allowing for more general material properties such as anisotropies, inhomogeneities, etc.

math-ph↗

arXiv2014

A characterization of boundary conditions yielding maximal monotone operators

We provide a characterization for maximal monotone realizations for a certain class of (nonlinear) operators in terms of their corresponding boundary data spaces. The operators under consideration naturally arise in the study of evolutionary problems in mathematical physics. We apply our abstract characterization result to Port-Hamiltonian systems and a class of frictional boundary conditions in the theory of contact problems in visco-elasticity.

math.FA↗