Popov, Alexander, and Mitko Michovski. “Assymptotic Solutions in the Ultrasonic Corrosion Control”. Ann. Sofia Univ. Fac. Math. and Inf. 88 (December 12, 1996): 341–346. Accessed November 24, 2024. https://annual.uni-sofia.bg/index.php/fmi/article/view/393.