Non-uniqueness of Weak Solutions to the Wave Map Problem

In this note we show that weak solutions to the wave map problem in the energy-supercritical dimension 3 are not unique. On the one hand, we find weak solutions using the penalization method introduced by Shatah and show that they satisfy a local energy inequality. On the other hand we build on a special harmonic map to construct a weak solution to the wave map problem, which violates this energy inequality. Finally we establish a local weak-strong uniqueness argument in the spirit of Struwe which we employ to show that one may even have a failure of uniqueness for a Cauchy problem with a stationary solution. We thus obtain a result analogous to the one of Coron for the case of the heat flow of harmonic maps.

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