1. I. G. MacDonald, Matematika,16, No. 4, 3–49 (1972).
2. I. N. Bernshtein, I. M. Gel'fand, and S. I. Gel'fand, Funktsional. Analiz i Ego Prilozhen.,5, No. 1, 1–9 (1971).
3. V. G. Kats, Izv. Akad. Nauk SSSR, Seriya Matem.,32, 1323–1367 (1968).
4. V. G. Kats, Trudy MIÉM, No. 5, 48–59 (1969).