In computability theory, a Friedberg numbering is a numbering (enumeration) of the set of all partial recursive functions that has no repetitions: each partial recursive function appears exactly once in the enumeration (Vereščagin and Shen 2003:30).
The existence of such numberings was established by Richard M. Friedberg in 1958 (Cutland 1980:78).
References
Friedberg numbering Wikipedia(Text) CC BY-SA