1. A. M. Savchuk and A. A. Shkalikov, Mat. Zametki 66 (6), 897 (1999) [Math. Notes 66 (5–6), 741 (1999)].
2. A. M. Savchuk and A. A. Shkalikov, Trudy Moskov. Mat. Obshch. 64, 159 (2003) [Trans. Moscow Math. Soc. 2003, 143 (2003)].
3. A. A. Vladimirov, Mat. Zametki 75 (6), 941 (2004) [Math. Notes 75 (5–6), 877 (2004)].
4. I.M. Guseinov and R. T. Pashaev, Dokl. Nat. Akad. Nauk Azerb. 62 (1), 7 (2006).
5. A. Goriunov and V. Mikhailets, Methods Funct. Anal. Topology 16 (2), 120 (2010).