Interior Schauder-Type Estimates for Higher-Order Elliptic Operators in Grand-Sobolev Spaces

In this paper an elliptic operator of the $m$-th order $L$ with continuous coefficients in the $n$-dimensional domain $\Omega \subset R^{n} $ in the non-standard Grand-Sobolev space $W_{q)}^{m} \left(\Omega \right)\, $ generated by the norm $\left\| \, \cdot \, \right\| _{q)} $ of the Grand-Lebesgue space $L_{q)} \left(\Omega \right)\, $ is considered. Interior Schauder-type estimates play a very important role in solving the Dirichlet problem for the equation $Lu=f$. The considered non-standard spaces are not separable, and therefore, to use classical methods for treating solvability problems in these spaces, one needs to modify these methods. To this aim, based on the shift operator, separable subspaces of these spaces are determined, in which finite infinitely differentiable functions are dense. Interior Schauder-type estimates are established with respect to these subspaces. It should be noted that Lebesgue spaces $L_{q} \left(G\right)\, $ are strict parts of these subspaces. This work is a continuation of the authors of the work \cite{28}, which established the solvability in the small of higher order elliptic equations in grand-Sobolev spaces.

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