MasmalievG. M., and KhanmamedovA. K. “On Conditions for the Discreteness of the Spectrum of a Semiinfinite Jacobi Matrix With Zero Diagonal”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 62, no. 2, Feb. 2010, pp. 285–288, https://umj.imath.kiev.ua/index.php/umj/article/view/2866.