B\^ocher type theorem for elliptic equations with drift perturbed L\'evy operator

A classical B\^ocher's theorem asserts that any positive harmonic function (with respect to the Laplacian) in the punctured unit ball can be expressed, up to the multiplication constant, as the sum of the Newtonian kernel and a positive function that is harmonic in the whole unit ball. This theorem expresses one of the fundamental results in the theory of isolated singularities and it can be viewed as a statement on the asymptotic behavior of positive harmonic functions near their isolated singularities. In the paper we generalize this results to drift perturbed L\'evy operators. We propose a new approach based on the probabilistic potential theory. It applies to L\'evy operators for which the resolvent of its perturbation is strongly Feller. In particular our result encompasses drift perturbed fractional Laplacians with any stability index bounded between zero and two - the method therefore applies to subcritical and supercritical cases.