On a bounded Lipschitz domain, the Sobolev trace operator is the continuous linear extension to of the boundary restriction of smooth functions. Continuity and density make this extension unique; its existence is the Sobolev trace theorem. The kernel prescribes a homogeneous Dirichlet boundary condition. Vanishing Sobolev trace on only part of the boundary instead gives the form domain for a mixed boundary condition, with the natural Neumann boundary condition on the remainder. On an assembly of congruent tiles, taking a Sobolev trace commutes with a constant matrix acting on the tile restrictions. This lets a transplantation theorem be proved on quadratic form domains even when corner eigenfunctions have singular classical derivatives.
In mathematics, particularly in the field of linear algebra and functional analysis, the trace operator is a function that assigns a single number to a square matrix (or more generally, to a linear operator). The trace of a matrix is defined as the sum of its diagonal elements.
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