Nauk, 34, No. 3, 137-168 (1979). 4. Yu. L. Ershov, "Positive equivalences," Algebra Logika, i0, 620-650 (1971). 5. G. N. Kobzev, "The complete ~#-degree," Algebra Logika, 13, 22-25 (1974). 6. G. N. Kobzev, "The semilattice of ~-degrees," Soobshcho Akad. Nauk Gruz. SSR, 90, 281283 (1978). 7. S. S. Marchenkov, "On the comparison of the upper semilattices of recursively enumerable n~-degrees and truth-table degrees," Mat. Zametki, 20, 19-26 (1976). 8. S. S. Marchenkov, "The tabular degrees of maximal sets," Mat. Zametki, 20, 373-381 (1976). 9. T. P. Cleave, "Some properties of recursively unseparable sets," Z. Math. Logik Grundl. Math., 16, 187-200 (1970). I0. A. H. Lachlan, "Two theorems on many-one degrees of recursively enumerable sets," Algebra Logika, II, 216-229 (1972). 11. D. A. Martin, "Classes of recursively enumerable sets and degrees of unsolvability," Z. Math. Logik Grundl. Math., 12, 295-310 (1966). 12. A. N. Degtev, "Relations between complete sets," Izv. Vyssh. Uchebn. Zaved., Mat., No. 5, 26-31 (1981).