“…When σ = 1/2 and 2, it recovers the prescribing mean curvature problem and fourth order Q-curvature problem, respectively; see [39,41] for brief reviews of the classical Nirenberg problem and its generalizations, as well as references in this area. Other studies on the fractional Nirenberg problem include Chen-Zheng [18], Abdelhedi-Chtioui-Hajaiej [1], Chen-Liu-Zheng [19], Guo-Nie-Niu-Tang [32], Liu-Ren [47], Niu-Tang-Wang [51], etc. The limiting case σ = n/2 is of particular interest; see Moser [50], Chang-Yang [14], Wei-Xu [57,58], Brendle [6,7], Da Lio-Martinazzi-Riviére [20], etc.…”