Paper detail

On Harnack inequality and Hölder regularity for isotropic unimodal Lévy processes

We prove the scale invariant Harnack inequality and regularity properties for harmonic functions with respect to an isotropic unimodal Lévy process with the characteristic exponent $ψ$ satisfying some scaling condition. We show sharp estimates of the potential measure and capacity of balls, and further, under the assumption of that $ψ$ satisfies the lower scaling condition, sharp estimates of the potential kernel of the underlying process. This allow us to establish the Krylov-Safonov type estimate, which is the key ingredient in the approach of Bass and Levin, that we follow.