“…Later Liu, Wooley, and Yu [10] showed that the exponent 2/5 can be improved to 3/8, and very recently, Harman and Kumchev [3] further reduced the exponent 3/8 to 5/14. There are also many other interesting approximations (see for example [1], [4], [9], [8], [13], [12] and their references).…”