Recent content by ubersweet987

Homework Statement Let (X,d) be a metric space and let A be a non-empty subset of X. Prove that A is open if and only if it can be written as the union of a family of open balls of the form Br(x) = {y ∈ X|d(x,y) < r} (the radius r may depend on the point x). Homework Equations...