Every metric space is a topological space in a natural manner.
The metric topology is the topology on (X,d) generated by the base Br(x)={y∈X:d(x,y)<r} (open balls). It is base because
- Union of Br(x) cover X (obviously)
- x∈Br1(x1)∩Br2(x2) then x∈Br(x)⊂Br1(x1)∩Br2(x2) with r=min{r1−d(x,x1),r2−d(x,x2)}
A base generates a topology on X that has, as open sets, all unions of elements of a base.
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