A hierarchy of Turing degrees a transfinite hierarchy of lowness notions in the computably enumerable degrees, unifying classes, and natural definability by Downey, R. G. (Rod G.), Greenberg, Noam 1974-

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Available for University of the Philippines System via IEEE Xplore. Click here to access

Cornerstones of undecidability by Rozenberg, Grzegorz

Degrees of unsolvability. by Sacks, Gerald E.