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Partitions of unity

In mathematics, a partition of unity on a topological space⁠X{\displaystyle X}⁠ is a set⁠R{\displaystyle R}⁠ of continuous functions from ⁠X{\displaystyle X}⁠ to the unit interval [0,1] such that for every point x∈X{\displaystyle x\in X}: there is a neighbourhood of ⁠x{\displaystyle x}⁠ where all but a finite number of the functions of ⁠R{\displaystyle R}⁠ are zero, andthe sum of all the function values at ⁠x{\displaystyle x}⁠ is 1, i.e., ∑ρ∈Rρ(x)=1.{\textstyle \sum _{ ho \in R} ho (x)=1.}Partitions of unity are useful because they often allow one to extend local constructions to the whole space.

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