##  [Boundary](/index.php/boundary) 

  ##  [Boundary](https://puremath.quantumdictionary.io/boundary-0) 

  

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**Pure Mathematics Dictionary**

 







 

 

 

 



 

 

 

 

Definition

The boundary ∂A of a subset A of a topological space X is the set of points whose every neighborhood intersects both A and its complement; equivalently ∂A = cl(A) \ int(A).