Smilijanow, A., and Z. Mitrov. “ON THE CORRECTNESS OF DIFFERENTIAL AND DIFFERENCE METHODS FOR ANALYSIS OF THE STABILITY OF CONTINUOUS RAILWAYS”. Ann. Sofia Univ. Fac. Math. And Inf., vol. 82, no. 1, Dec. 1991, pp. 5-12, https://ftl5.uni-sofia.bg./index.php/fmi/article/view/518.