Kalimbetov, B.T., and B.I. Eskaraeva. “Contrast Structure in Equations With Zero Spectrum of Limit Operator and Irreversible Spectral Value of the Kernel”. Bulletin of the Karaganda University. Mathematics Series 78, no. 2 (June 29, 2015): 56–64. Accessed December 9, 2025. https://mts.buketov.edu.kz/mathematics-vestnik/article/view/24.