# Lipschitz and / or piecewise smooth boundary

Let $$\Omega_3 = \overline{Q_1 \cup Q_2}$$ with $$Q_1 = (-1, 3) \times (0,2) \times (-1, 0)$$ and $$Q_2 = (0,2) \times (-1, 3) \times (0,1)$$

Here is what I've done until now: $$\Omega_3$$ can be imagines as two cuboids which are on top of each other. So, geometrically, one can see that the boundary is smooth. But how can this be shown mathematically?