A. M. Kholkin
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Spectral singularities of differential operator with triangular matrix coefficients
MFAT 19 (2013), no. 3, 260-267
For a non-selfadjoint Sturm-Liouville operator with a triangular matrix potential growing at infinity, we construct an example of such an operator having spectral singularities.
Sturm type oscillation theorems for equations with block-triangular matrix coefficients
A. M. Kholkin, F. S. Rofe-Beketov
MFAT 18 (2012), no. 2, 176-188
A relation is established between spectral and oscillation properties of the problem on a finite interval and a semi-axis for second order differential equations with block-triangular matrix coefficients.