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Jincheng Yang

Institute for Advanced Study

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Research Blog

Covering Lemma

2020, April 11

Vitali Covering Lemma

Statement

Let E be a measurable set in \mathbb R ^n, and let \{ B _\alpha \} _{\alpha \in \Lambda} be a collection of balls that covers E with uniformly bounded radii. Then there exists a pairwise disjoint subcollection \{ B _{\alpha _j} \} such that \bigcup _j 5 B _{\alpha _j} covers E.

Algorithm

In j-th iteration, we

  • Find the largest ball B _{\alpha _j}
  • Remove every B _\beta that intersects with B _{\alpha _j}

Besicovitch (Безико́вич) Covering Lemma

Statement

Let E be a measurable set in \mathbb R ^n, and Let \{ Q _\alpha \} _{\alpha \in \Lambda} be a collection of cubes with uniformly bounded radii, such that for every x \in E there exists at least one Q _\alpha centering at x. Then there exists a finitely overlapping subcover of E.

Algorithm

In j-th iteration, we

  • Find the largest cube Q _{\alpha _j}
  • Remove every Q _\beta that centers within Q _{\alpha _j}

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