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S. C. Kleene, Introduction to metamathematics, Van Nostrand, New York, 1952. MR 0051790 (14:525m)

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S. C. Kleene and E. L. Post, The upper semi-lattice of degrees of recursive unsolvability, Ann. of Math. (2) 59 (1954), 379-407. MR 0061078 (15:772a)

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A. A. Muchnik, Negative answer to the problem of reducibility of algorithms, Dokl. Akad. Nauk SSSR 108 (1956), 194-197. (Russian)

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Gerald E. Sacks, The universality of the recursively enumerable degrees, Abstract No. 60T-14, Notices Amer. Math. Soc. 7 (1960), 996.

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J. R. Shoenfield, On degrees of unsolvability, Ann. of Math. (2) 69 (1959), 644-653. MR 0105355 (21:4097)

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Clifford Spector, On degrees of recursive unsolvability, Ann. of Math. (2) 64 (1956), 581-592. MR 0082457 (18:552d)