Songsuwan, W. ., and N. . Wattanasakulpong. “Amplitude Analysis of Functionally Graded Beams under Linear Decreasing and Exponential Loads”. Journal of Research and Applications in Mechanical Engineering, vol. 8, no. 1, June 2020, pp. 1-10, https://ph01.tci-thaijo.org/index.php/jrame/article/view/217970.