The Boundary Harnack Principle and the 3G Principle in Fractal-Type Spaces

We prove a generalized version of the $3G$ Principle for Green’s functions on bounded inner uniform domains in a wide class of Dirichlet spaces. In particular, our results apply to higher-dimensional fractals such as Sierpinski carpets in ${\mathbf{R}}^n$, $n\ge 3$, as well as generalized fractal-type spaces that do not have a well-defined Hausdorff dimension or walk dimension. This yields new instances of the $3G$ Principle for these spaces. We also discuss applications to Schrödinger operators.