1. D. Waterman, Stud. Math. 44(1), 107–117 (1972).
2. A. A. Saakyan, Izv. Akad. Nauk Arm. SSR 21, 517–529 (1986).
3. A. I. Sablin, Izv. Vyssh. Uchebn. Zaved. Mat., No. 10, 66–68 (1987).
4. A. N. Bakhvalov, Mat. Sb. 193(12), 3–20 (2002).
5. A. N. Bakhvalov, Izv. Akad. Nauk, Ser. Mat. 67(6), 3–22 (2003).