“…The interest in studying such problems was stimulated by their application in mathematical physics, more precisely in elastic mechanics [25], electrorheological fluids and stationary thermo-rheological viscous flows of non-Newtonian fluids, image processing [8,12,19,20] and the mathematical description of the processes filtration of an idea barotropic gas through a porous medium [3,7]. Many results have been obtained on this kind of problems, for instance, we here cite [1,4,10,13,14,16,18,21,22].…”