“…For background and further discussion relating to the material covered here the reader is referred to the texts [10,18,27,30,34,36,42] and the references therein. For recent results and literature on gradient estimates see [3, 8, 13, 15, 19-21, 31, 35, 37, 39, 40] and for closely related topics see [1,2,7,11,14,16,23,25,26,28,32,38].…”