Auciello, N.M. “A Study of the Numerical Convergence of Rayleigh-Ritz Method for the Free Vibrations of Cantilever Beam of Variable Cross-Section With Tip Mass”. Engineering Transactions, vol. 44, no. 3-4, Dec. 1996, pp. 375-88, https://et.ippt.pan.pl/index.php/et/article/view/1469.