Construct a function $\rho \in \mathcal{C}^\infty(\mathbb{R})$, constant equal to 1 on $[-1,1]$ and constant equal to 0 on $\mathbb{R} \setminus [-2,2]$.
Construct a function $\rho \in \mathcal{C}^\infty(\mathbb{R})$, constant equal to 1 on $[-1,1]$ and constant equal to 0 on $\mathbb{R} \setminus [-2,2]$.