[1]
A. M. Kholkin, “Spectral singularities of differential operator with triangular matrix coefficients”, MFAT, vol. 19, no. 3, pp. 260–267, Sep. 2013, Accessed: Mar. 30, 2026. [Online]. Available: https://zen.imath.kiev.ua/index.php/mfat/article/view/551