Extensions for Systems of Conservation Laws

TL;DR

This paper introduces a geometric approach to analyze extensions in hyperbolic conservation laws by studying the eigenvector lengths, providing a systematic method for existence and classification of extensions across different systems.

Contribution

It develops a geometric framework focusing on eigenvector lengths to determine extensions, applicable to all systems sharing the same eigen-frame, and classifies possibilities for systems of three equations and larger.

Findings

Systematic method for existence of extensions

Complete classification for 3-equation systems

Applicability to rich hyperbolic systems of any size

Abstract

Extensions (entropies) play a central role in the theory of hyperbolic conservation laws by providing intrinsic selection criteria for weak solutions. For a given hyperbolic system u_t+f(u)_x=0, a standard approach is to analyze directly the second order PDE system for the extensions. Instead we find it advantageous to consider the equations satisfied by the lengths beta^i of the right eigenvectors r_i the Jacobian matrix Df, as measured with respect to the inner product defined by an extension. Our geometric formulation provides a natural and systematic approach to existence of extensions. By prescribing the eigen-fields r_i our results automatically apply to all systems with the same eigen-frame. The equations for the lengths beta^i form a first order algebraic-differential system (the beta-system) to which standard integrability theorems can be applied. The size of the set of extensions follows by determining the number of free constants and functions present in the general solution to the beta-system. We provide a complete breakdown of the various possibilities for systems of three equations, as well as for rich hyperbolic systems of any size.