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A bounded open Jordan set has an increasing exhaustion by compact finite unions of grid rectangles with vanishing content remainder
Statement
Let and let be bounded, open, and Jordan measurable. There are compact Jordan sets each a finite union of closed grid rectangles, such that every compact lies in some and
Facts & Assumptions
Given: Bounded open Jordan set .
A bounded set is Jordan measurable exactly when its boundary has content zero (A bounded set in is Jordan measurable iff its boundary is null, equivalently of content zero).
A finite cube cover can be replaced by sufficiently fine grid cells with controlled total volume (A finite rectangle cover admits grid control with arbitrarily small volume excess).
Jordan content is finitely additive on interior-disjoint Jordan pieces (Jordan content is finitely additive when the overlap has content zero).
Proof
Enclose in a rectangle and choose nested dyadic grids whose meshes tend to zero. Let be the union of every closed cell of the th grid that is contained in . Only finitely many cells occur. Every child of a retained cell is retained, so ; each is compact, Jordan, and contained in .
If compact , the distance from to the closed complement of is positive. Once the mesh diameter is smaller than that distance, every grid cell meeting is contained in , so .
Every unretained cell meeting also meets a mesh-sized neighborhood of . By [L1], that boundary has content zero; [L2] therefore makes the total volume of all such cells arbitrarily small for fine enough grids. Finite additivity [L3] bounds by that volume, proving the limit.
Depends on
- Grid partitions of a rectangle in $\mathbb{R}^m$, their cells, refinements and mesh
- Jordan inner and outer content and Jordan measurable bounded sets in $\mathbb{R}^m$
- A bounded set in $\mathbb{R}^m$ is Jordan measurable iff its boundary is null, equivalently of content zero
- A finite rectangle cover admits grid control with arbitrarily small volume excess
- Jordan content is finitely additive when the overlap has content zero
Used by
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Sources
- A. Leibman, Multidimensional Real Analysis, §5.5 (standard reference, not scraped)