Kim, N.I., T.A. Huynh, Q.X. Lieu, and J. Lee. 2018. “NURBS-Based Optimization of Natural Frequencies for Bidirectional Functionally Graded Beams”. Archives of Mechanics 70 (4): 337-64. https://am.ippt.pan.pl/index.php/am/article/view/v70p337.