Linear PDEs in critical regularity spaces: Hierarchical construction of their nonlinear solutions

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Eitan Tadmor, University of Maryland

We construct uniformly bounded solutions of the equations div(U)=f and curl(U)=f, for general f in the critical regularity spaces Ld(Rd) and, respectively, L3(R3). The study of these equations was motivated by recent results of Bourgain & Brezis. The equations are linear but construction of their solutions is not. Our constructions are, in fact, special cases of a rather general framework for solving linear equations, L(U)=f, covered by the closed range theorem. The solutions are realized in terms of nonlinear hierarchical representations, U=sum(uj), which we introduced earlier in the context of image processing. The uj's are constructed recursively as proper minimizers, yielding a multi-scale decomposition of the solutions U.