“…For example, when n = 2 we have µ = δ (1,0) + δ (0,1) + δ (−1,0) + δ (0,−1) (up to a multiplicative constant). The regularity of solutions to Lu = f (or Lu = 0) for operators L like (1.3), (1.1), or related ones, has been widely investigated; see the works by Bass, Kassmann, Schwab, Silvestre, Sztonyk, and Bogdan, among others [1,23,22,3,39,21,2,37,5,6,7,9,24]. A typical assumption in some of these results is that 0 < c ≤ a(θ) ≤ C in S n−1 .…”