Second order elliptic operators with periodic coefficients: Criticality theory, perturbations, and positive harmonic functions (Q1891818)

The author studies the existence and characterization of positive harmonic functions for second order elliptic operators with periodic coefficients, the behavior of the generalized principal eigenvalue under perturbations, and a number of questions related to the above mentioned problems. All these results, known to hold in the case of constant coefficients, are proved in the periodic case.