Originally Posted by

**hjortur** Well I guess this is not standard notation but here goes:

Let $\displaystyle A$ be a set in $\displaystyle \mathbb{R}^n$, then $\displaystyle \partial A$ is the boundry of A.

$\displaystyle \overline{A}=A\cup\partial A$

and

$\displaystyle \overset{\circ}{A}=A\setminus\partial A$

Now prove that

$\displaystyle \overset{\circ}{\overset{\_}{\overset{\circ}{\over set{\_}{A}}}}=\overset{\circ}{\overset{\_}{A}}$

and

$\displaystyle \overset{\_}{\overset{\circ}{\overset{\_}{\overset {\circ}{A}}}}=\overset{\_}{\overset{\circ}{A}}$

I just have no idea how to do this one. Any hints greatly appriciated.