Minkowski content

The Minkowski content (named after Hermann Minkowski), or the boundary measure, of a set is a basic concept that uses concepts from geometry and measure theory to generalize the notions of length of a smooth curve in the plane, and area of a smooth surface in space, to arbitrary measurable sets. 
It is typically applied to fractal boundaries of domains in the Euclidean space, but it can also be used in the context of general metric measure spaces.
It is related to, although different from, the Hausdorff measure.
Definition.
For formula_1, and each integer "m" with formula_2, the "m"-dimensional upper Minkowski content is
and the "m"-dimensional lower Minkowski content is defined as
where formula_5 is the volume of the ("n"−"m")-ball of radius r and formula_6 is an formula_7-dimensional Lebesgue measure. 
If the upper and lower "m"-dimensional Minkowski content of "A" are equal, then their common value is called the Minkowski content "M""m"("A").