“…And the situation is even worse in the spherical case that the uniqueness result does not hold in general ([13, Theorem 1.5]). As a consequence, the spherical case of Question 2.1 is still widely open although many mathematicians had attacked or have been investigating it by using various methods and obtained a good understanding of the question ( [28,6,9,13,15,16,17,18,19,20,21,32,33,34,35,39,44,45,47,48]).…”