“…The other important issue about the topology optimization for eigenfrequencies is the sensitivity analysis of eigenfrequencies, especially of multiply repeated eigenfrequencies, with respect to a design variable. Last decades have witnessed the development of many methods for the sensitivity analysis of eigenfrequencies, for example, the methods by computing the sensitivities of eigenvalues [19,31] and the methods via a smooth approximation [4,[32][33][34].…”