Harnack inequality for a class of degenerate elliptic operators
  We prove a Harnack inequality for a class of two-weight degenerate elliptic operators. The metric distance is induced by continuous Grushin-type vector fields. It is not know whether there exist cutoffs fitting the metric balls. This obstacle is bypassed by means of a covering argument that allows the use of rectangles in the Moser iteration. 
J. D. Fernandes, J. Groisman, S. T. Melo