Andrea Walther

Modelling and Analysis of an Inverse Parameter Identification Problem in Piezoelectricity

  • Raphael Kuess
  • Daniel Walter
  • Andrea Walther
    • Categories Infinite Dimensional Optimization, Nonlinear Systems and Least-Squares, Systems governed by Differential Equations Optimization Tags , , , , ,

    Piezoelectric material behavior is mathematically described by coupled hyperbolic-elliptic partial differential equations (PDEs) governing mechanical displacement and electrical potential. This paper presents advancements in the theory of identifying material parameters in piezoelectric PDEs. We focus on modeling and analyzing the inverse problem assuming matrix-valued Sobolev-Bochner parameters to encompass a time and space-dependent setting and thus … Read more

    Sensitivity-informed identification of temperature-dependent piezoelectric material parameters

  • Raphael Kuess
  • Olga Friesen
  • Leander Claes
  • Andrea Walther
    • Categories Applications - Science and Engineering, Control Applications, Nonlinear Optimization Tags , , ,

    An accurate characterization of temperature-dependent material parameters of piezoceramics is crucial for the design and simulation of reliable sensors and actuators. This characterization is typically formulated as an ill-posed inverse problem, which is challenging to solve not only because of its ill-posedness, but also because of parameter sensitivities, which vary by several orders of magnitude … Read more

    On regularized structure exploiting Quasi-Newton methods for inverse problems

  • Raphael Kuess
  • Andrea Walther
    • Categories Infinite Dimensional Optimization, Nonlinear Systems and Least-Squares Tags , , , ,

    This paper introduces a regularized, structure-exploiting Powell-Symmetric-Broyden (RSE-PSB) method under modified secant conditions for solving ill-posed inverse problems in both infinite dimensional and finite dimensional settings. The approximation of the symmetric, yet potentially indefinite, second-order term, which is neglected by standard Levenberg-Marquardt (LM) approaches, integrates regularization and structure exploitation directly within the Quasi-Newton (QN) framework, … Read more

    How Stringent is the Linear Independence Kink Qualification in Abs-Smooth Optimization?

  • Franz Bethke
  • Andrea Walther
    • Categories Nonsmooth Optimization Tags

    Abs-smooth functions are given by compositions of smooth functions and the evaluation of the absolute value. The linear independence kink qualification (LIKQ) is a fundamental assumption in optimization problems governed by these abs-smooth functions, generalizing the well-known LICQ from smooth optimization. In particular, provided that LIKQ holds it is possible to derive optimality conditions for … Read more

    Stochastic Aspects of Dynamical Low-Rank Approximation in the Context of Machine Learning

  • Arsen Hnatiuk
  • Lisa Kusch
  • Jonas Kusch
  • Nicolas R. Gauger
  • Andrea Walther
    • Categories Data Science Theory, Nonlinear Optimization, Optimization in Data Science Tags , , , ,

    The central challenges of today’s neural network architectures are the prohibitive memory footprint and training costs associated with determining optimal weights and biases. A large portion of research in machine learning is therefore dedicated to constructing memory-efficient training methods. One promising approach is dynamical low-rank training (DLRT), which represents and trains parameters as a low-rank … Read more

    On a Frank-Wolfe Approach for Abs-smooth Functions

  • Timo Kreimeier
  • Sebastian Pokutta
  • Andrea Walther
  • Zev Woodstock
    • Categories Constrained Nonlinear Optimization, Nonsmooth Optimization, Other Topics Tags , , , ,

    We propose an algorithm which appears to be the first bridge between the fields of conditional gradient methods and abs-smooth optimization. Our problem setting is motivated by various applications that lead to nonsmoothness, such as $\ell_1$ regularization, phase retrieval problems, or ReLU activation in machine learning. To handle the nonsmoothness in our problem, we propose … Read more

    A Semismooth Conjugate Gradients Method — Theoretical Analysis

  • Franz Bethke
  • Andreas Griewank
  • Andrea Walther
    • Categories Nonsmooth Optimization Tags , , ,

    In large scale applications, deterministic and stochastic variants of Cauchy’s steepest descent method are widely used for the minimization of objectives that are only piecewise smooth. In this paper we analyse a  deterministic descent method based on the generalization of rescaled conjugate gradients proposed by Philip Wolfe in 1975 for objectives that are convex. Without … Read more

    An active signature method for constrained abs-linear minimization

  • Timo Kreimeier
  • Andrea Walther
  • Andreas Griewank
    • Categories Nonsmooth Optimization Tags , , , , ,

    In this paper we consider the solution of optimization tasks with a piecewise linear objective function and piecewise linear constraints. First, we state optimality conditions for that class of problems using the abs-linearization approach and prove that they can be verified in polynomial time. Subsequently, we propose an algorithm called Constrained Active Signature Method that … Read more

    A Structure Exploiting Algorithm for Non-Smooth Semi-Linear Elliptic Optimal Control Problems

  • Stephan Schmidt
  • Olga Weiß
  • Andrea Walther
    • Categories Infinite Dimensional Optimization, Nonsmooth Optimization

    We investigate optimization problems with a non-smooth partial differential equation as constraint, where the non-smoothness is assumed to be caused by Nemytzkii operators generated by the functions abs, min and max. For the efficient as well as robust solution of such problems, we propose a new optimization method based on abs-linearization, i.e., a special handling … Read more

    An Algorithm for Piecewise Linear Optimization of Objective Functions in Abs-normal Form

  • Andreas Griewank
  • Andrea Walther
    • Categories Nonsmooth Optimization Tags , , , , , , ,

    In the paper [11] we derived first order (KKT) and second order (SSC) optimality conditions for functions defined by evaluation programs involving smooth elementals and absolute values. For this class of problems we showed in [12] that the natural algorithm of successive piecewise linear optimization with a proximal term (SPLOP) achieves a linear or even … Read more