1. Two recursively enumerable sets of incomparable degrees of unsolvability;Friedberg,1957
2. The upper semi-lattice of degrees of recursive unsolvability;Kleene;Ann. of Math.,1954
3. Lower bounds for pairs of r.e. degrees;Lachlan;Proc. London Math. Soc.,1966
4. Distributive initial segments of the degrees of unsolvability;Lachlan;Z. Math. Logik Grundlagen Math.,1968
5. Embedding nondistributive lattices in the recursively enumerable degrees;Lachlan,1972