Le, Bui Hai, and Tran Minh Thuy. 2017. “Optimal Design for Eigen-Frequencies of a Longitudinal Bar Using Pontryagin’s Maximum Principle Considering the Influence of Concentrated Mass”. Vietnam Journal of Mechanics 39 (1). Hanoi, Vietnam:1-12. https://doi.org/10.15625/0866-7136/6058.