Khalina, K. S. “Boundary Controllability Problems for the Equation of Oscillation of an Inhomogeneous String on a Semiaxis”. Ukrains’kyi Matematychnyi Zhurnal, vol. 64, no. 4, Apr. 2012, pp. 525-41, https://umj.imath.kiev.ua/index.php/umj/article/view/2593.