CONTENTS Preliminaries........................................................................................................ 5 1. Auxiliary results......................................................................................................... 13 2. The second order equations.................................................................................. 14 3. Some properties of Sobolev and Besov spaces................................................ 20 4. Classes $Λ^α(G, H)$, 0 < a ≤ 1............................................................................ 21 5. The case of Lipschitz characteristics................................................................... 26 6. Existence of second partial derivatives and its consequences...................... 29 7. Local boundedness of the Jacobian.................................................................... 33 8. Smoothness.............................................................................................................. 41 References.................................................................................................................... 44
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We give an example relating to the regularity properties of mappings with finite distortion. This example suggests conditions to be imposed on the distortion function in order to avoid "cavitation in measure".
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We establish an interpolation theorem for a class of nonlinear operators in the Lebesgue spaces $ℒ^{s}(ℝⁿ)$ arising naturally in the study of elliptic PDEs. The prototype of those PDEs is the second order p-harmonic equation $div|∇u|^{p-2∇} u = div 𝔣$. In this example the p-harmonic transform is essentially inverse to $div(|∇|^{p-2}∇)$. To every vector field $𝔣 ∈ ℒ^{q}(ℝⁿ,ℝⁿ)$ our operator $ℋ_{p}$ assigns the gradient of the solution, $ℋ_{p}𝔣 = ∇u ∈ ℒ^{p}(ℝⁿ,ℝⁿ)$. The core of the matter is that we go beyond the natural domain of definition of this operator. Because of nonlinearity our arguments require substantial innovations as compared with the classical interpolation theory of Riesz, Thorin and Marcinkiewicz. The subject is largely motivated by recent developments in geometric function theory.
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