In real analysis, a branch of mathematics, Cousin's theorem states that:
If for every point of a closed region (in modern terms, "closed and bounded") there is a circle of finite radius (in modern term, a "neighborhood"), then the region can be divided into a finite number of subregions such that each subregion is interior to a circle of a given set having its center in the subregion.This result was proved and established by Pierre Cousin, a student of Henri Poincaré, in 1895, and it is an extension of the original Heine–Borel theorem on compactness for arbitrary covers of any compact subsets of
Nowadays, it is stated as:
LetFurther, Cousin's theorem is mainly only used in Henstock–Kurzweil integral and is often called Fineness Theorem or Cousin's lemma. It can be stated as:
If I := [a, b] ⊆ Rn is a nondegenerate compact interval and δ is any gauge defined on I, then there always exists a tagged partition of I that is δ-fine.