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Journal of Computational Dynamics

October 2021 , Volume 8 , Issue 4

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Computing Reeb dynamics on four-dimensional convex polytopes
Julian Chaidez and Michael Hutchings
2021, 8(4): 403-445 doi: 10.3934/jcd.2021016 +[Abstract](680) +[HTML](365) +[PDF](572.97KB)

We study the combinatorial Reeb flow on the boundary of a four-dimensional convex polytope. We establish a correspondence between "combinatorial Reeb orbits" for a polytope, and ordinary Reeb orbits for a smoothing of the polytope, respecting action and Conley-Zehnder index. One can then use a computer to find all combinatorial Reeb orbits up to a given action and Conley-Zehnder index. We present some results of experiments testing Viterbo's conjecture and related conjectures. In particular, we have found some new examples of polytopes with systolic ratio \begin{document}$ 1 $\end{document}.

Tracking the critical points of curves evolving under planar curvature flows
Eszter Fehér, Gábor Domokos and Bernd Krauskopf
2021, 8(4): 447-494 doi: 10.3934/jcd.2021017 +[Abstract](609) +[HTML](284) +[PDF](5208.39KB)

We are concerned with the evolution of planar, star-like curves and associated shapes under a broad class of curvature-driven geometric flows, which we refer to as the Andrews-Bloore flow. This family of flows has two parameters that control one constant and one curvature-dependent component for the velocity in the direction of the normal to the curve. The Andrews-Bloore flow includes as special cases the well known Eikonal, curve-shortening and affine shortening flows, and for positive parameter values its evolution shrinks the area enclosed by the curve to zero in finite time. A question of key interest has been how various shape descriptors of the evolving shape behave as this limit is approached. Star-like curves (which include convex curves) can be represented by a periodic scalar polar distance function \begin{document}$ r(\varphi) $\end{document} measured from a reference point, which may or may not be fixed. An important question is how the numbers and the trajectories of critical points of the distance function \begin{document}$ r(\varphi) $\end{document} and of the curvature \begin{document}$ \kappa(\varphi) $\end{document} (characterized by \begin{document}$ dr/d\varphi = 0 $\end{document} and \begin{document}$ d\kappa /d\varphi = 0 $\end{document}, respectively) evolve under the Andrews-Bloore flows for different choices of the parameters.

We present a numerical method that is specifically designed to meet the challenge of computing accurate trajectories of the critical points of an evolving curve up to the vicinity of a limiting shape. Each curve is represented by a piecewise polynomial periodic radial distance function, as determined by a chosen mesh; different types of meshes and mesh adaptation can be chosen to ensure a good balance between accuracy and computational cost. As we demonstrate with test-case examples and two longer case studies, our method allows one to perform numerical investigations into subtle questions of planar curve evolution. More specifically — in the spirit of experimental mathematics — we provide illustrations of some known results, numerical evidence for two stated conjectures, as well as new insights and observations regarding the limits of shapes and their critical points.

Classification with Runge-Kutta networks and feature space augmentation
Elisa Giesecke and Axel Kröner
2021, 8(4): 495-520 doi: 10.3934/jcd.2021018 +[Abstract](644) +[HTML](258) +[PDF](5286.25KB)

In this paper we combine an approach based on Runge-Kutta Nets considered in [Benning et al., J. Comput. Dynamics, 9, 2019] and a technique on augmenting the input space in [Dupont et al., NeurIPS, 2019] to obtain network architectures which show a better numerical performance for deep neural networks in point and image classification problems. The approach is illustrated with several examples implemented in PyTorch.

2021 CiteScore: 1.7




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