Hi all again.

I have a very simple question that I cannot find a clue.

$\displaystyle \mathrm{Ext}(A):=(\overline{A})^{c}$ (external of $\displaystyle A$), where $\displaystyle \overline{\cdot}$ is the closure and $\displaystyle \cdot^{c}$ is the complementary.

$\displaystyle \partial A:=\overline{A}\backslash\overset{\circ}{A}$ (boundary of $\displaystyle A$).

Prove that $\displaystyle \partial\mathrm{Ext}(A)=\partial A$.

Thanks.