“…Problem (1.1) is also well known as the Ky Fan inequality early studied in [9]. In recent years, this problem has received a lot of attention by many authors, for instance, see [1,2,6,10,11,12,13,14,15,17,23,26,27,28,32]. This interest can be that, as be seen, it unifies all the aforementioned particular problems in a convenient way.…”