1. Computable Structures and the Hyperarithmetical Hierarchy;Ash,2000
2. The Ershov hierarchy;Arslanov,2011
3. Properly Σ20 enumeration degrees;Cooper;Z. Math. Log. Grundl. Math.,1988
4. On minimal pairs of enumeration degrees;Cooper;J. Symbolic Logic,1985
5. Enumeration reducibility, nondeterministic computations and relative computability of partial functions;Cooper,1990