Harnack inequalities, exponential separation, and perturbations of principal Floquet bundles for linear parabolic equations Juraj Húska; Peter Poláčik; Mikhail V. Safonov — 2007 Annales de l'I.H.P. Analyse non linéaire
Doubling properties for second order parabolic equations. Safonov, Mikhail V.; Yuan, Yu — 1999 Annals of Mathematics. Second Series