Hopf-Oleinik lemma for elliptic equations in double divergence form

We establish, for the first time, a Zaremba-Hopf-Oleinik type boundary point lemma for uniformly elliptic partial differential equations in double divergence form, also known as stationary Fokker-Planck-Kolmogorov equations. As an application, we derive sharp two-sided estimates for the Green’s function associated with second-order elliptic equations in non-divergence form in $C^{1,\alpha }$ domains.