Finding Solution Subspaces of the Laplace and Heat Equations Isometric to Spaces of Real Functions, and Some of Their Applications

We single out subspaces of harmonic functions in the upper half-plane coinciding with spaces of convolutions with the Abel–Poisson kernel and subspaces of solutions of the heat equation coinciding with spaces of convolutions with the Gauss–Weierstrass kernel that are isometric to the corresponding spaces of real functions defined on the set of real numbers. It is shown that, due to isometry, the main approximation characteristics of functions and function classes in these subspaces are equal to the corresponding approximation characteristics of functions and function classes of one variable.