A strong reducibility relation between partial numberings is introduced which is such that the reduction function transfers
exactly the numbers which are indices under the numbering to be reduced into corresponding indices of the other numbering. The degrees of partial numberings of a given set with respect to this relation form an upper semilattice.
In addition, Ershov
s completion construction for total numberings is extended to the partial case: every partially numbered set can be embedded in a set which results from the given set by adding one point and which is enumerated by a total and complete numbering. As is shown, the degrees of complete numberings of the extended set also form an upper semilattice. Moreover, both semilattices are isomorphic.
This is not so in the case of the usual, weaker reducibility relation for partial numberings which allows the reduction function to transfer arbitrary numbers into indices.
Keywords Partial numberings - Total numberings - Complete numberings - Reducibility - Degree structure
This research has partially been supported by INTAS under grant 00-499 
Computability in Hierarchies and Topological Spaces.
Mathematics Subject Classification (2000): 03D45