“…It is important to mention that many mathematicians worked hard on this problem, making valuable progress with respect to the problem. For example: [4], [9], [12], [14], [16], [17], [18], [23], [24], [26], [27], [30], [36], [37], [38], [39], [40], [41] etc. Unfortunately it is not possible to comment on and cite all the existing works.…”