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## Homework Statement

Let [tex](X,d)[/tex] be a metric space and let [tex]A \subseteq X[/tex]. Denote the interior of [tex]A[/tex] by [tex]A^o[/tex].

## Homework Equations

Prove that if [tex]A[/tex] is open or closed, then [tex](\partial A)^o = \varnothing[/tex]. (Is this still true if [tex]A[/tex] is not open or closed?)

## The Attempt at a Solution

I don't even know what [tex]\partial A[/tex] means. Can anyone tell me what this represents?