arXiv subjects

Scott B. Lindstrom

Publications and source records attributed to Scott B. Lindstrom.

21 records · Page 2Linked to original sources

arXiv2018

Dynamics of the Douglas-Rachford Method for Ellipses and p-Spheres

We expand upon previous work that examined behavior of the iterated Douglas-Rachford method for a line and a circle by considering two generalizations: that of a line and an ellipse and that of a line together with a $p$-sphere. With computer assistance we discover a beautiful geometry that illustrates phenomena which may affect the behavior of the iterates by slowing or inhibiting convergence for feasible cases. We prove local convergence near feasible points, and---seeking a better understanding of the behavior---we employ parallelization in order to study behavior graphically. Motivated by the computer-assisted discoveries, we prove a result about behavior of the method in infeasible cases.

math.FA↗

arXiv2018

Proximal Averages for Minimization of Entropy Functionals

In their 2016 article `Meetings with Lambert $\mathcal{W}$ and Other Special Functions in Optimization and Analysis', Borwein and Lindstrom considered the minimization of an entropy functional utilizing the weighted average of the negative Boltzmann--Shannon entropy $x\mapsto x\log x -x$ and the energy $x \mapsto x^2/2$; the solution employed the Fenchel-Moreau conjugate of the sum. However, in place of a traditional arithmetic average of two convex functions, it is also, perhaps even more, natural to use their proximal average. We explain the advantages and illustrate them by computing the analogous proximal averages for the negative entropy and energy. We use the proximal averages to solve entropy functional minimization problems similar to those considered by Borwein and Lindstrom, illustrating the benefits of using a true homotopy. Through experimentation, we discover computational barriers to obtaining solutions when computing with proximal averages, and we demonstrate a method which appears to remedy them.

math.OC↗

arXiv2018

The Douglas--Rachford algorithm for a hyperplane and a doubleton

The Douglas--Rachford algorithm is a popular algorithm for solving both convex and nonconvex feasibility problems. While its behaviour is settled in the convex inconsistent case, the general nonconvex inconsistent case is far from being fully understood. In this paper, we focus on the most simple nonconvex inconsistent case: when one set is a hyperplane and the other a doubleton (i.e., a two-point set). We present a characterization of cycling in this case which --- somewhat surprisingly --- depends on whether the ratio of the distance of the points to the hyperplane is rational or not. Furthermore, we provide closed-form expressions as well as several concrete examples which illustrate the dynamical richness of this algorithm.

math.OC↗