1. V. M. Adamjan and H. Langer, J. Operator Theory 33 (2), 259 (1995).
2. A. A. Shkalikov and V. E. Vladykina, Math. Notes 98 (5–6), 891 (2015).
3. V. Adamjan, H. Langer, and M. Langer, J. Differential Equations 171 (2), 315 (2001).
4. E. P. Bogomolova, Differ. Uravn. 21 (11), 1843 (1985) [Differ. Equations 21 (11), 1229 (1985)].
5. H. Langer, A. Markus, and V. Matsaev, in Oper. Theory Adv. Appl., Operator Theory and Related Topics, Vol. II (Birkhäuser, Basel, 2000), Vol. 118, pp. 255–285.