Dairbayeva, G. M., and L. N. Temirbekova. “Numerical Analogue of the Gelfand-Levitan on a Finite Interval”. Journal of Mathematics, Mechanics and Computer Science, vol. 76, no. 1, Mar. 2013, pp. 60-70, https://bm.kaznu.kz/index.php/kaznu/article/view/88.