Inspired by the two posts which are linked bellow we ask the following question:

**Question:** For a vector field $X$ on the plane we define the differential operator $D$ on $C^{\infty}(\mathbb{R}^2)$ with $D(f)=(\Delta\circ L_X-L_X\circ \Delta)(f)$ where $\Delta$ is the standard Laplacian.

Is there a vector field $X$ on the plane with $2$ nested closed orbits $\gamma_1 \subset \gamma_2$ such that there exist a smooth function $f$ for which $D(f)$ does not vanish on the closur of the amnular region surrounded by $\gamma_1$ and $\gamma_2$?

Elliptic operators corresponds to non vanishing vector fields

**Remark:** The first words of the titles of this post is inspired by a paper by C.C. Pugh and J.P Francois " Keeping track of Limit cycles"