The Green's function of $(\nabla^2 -m^2)$ is
$$\Delta(x,y)=\int d^4k \frac{e^{ik\cdot(x-y)}}{k^2+m^2+i\varepsilon}$$
so that $(\nabla^2 -m^2)\Delta(x,y)=\delta^{(4)}(x-y)$
Is it possible to find the Green's function of:
$$f(x)(\nabla^2 -m^2)$$
where $f(x)$ is an arbitrary function? Such a Green's function $G(x,y)$ satisfies:
$$f(x)(\nabla^2 -m^2)G(x,y)=\delta^{(4)}(x-y)$$