The Journal of Symbolic Logic

Research Article

Three theorems on recursive enumeration. I. Decomposition. II. Maximal set. III. Enumeration without duplication

Richard M. Friedberg

Harvard University

In this paper we shall prove three theorems about recursively enumerable sets. The first two answer questions posed by Myhill [1].

The three proofs are independent and can be presented in any order. Certain notations will be common to all three. We shall denote by “Re ” the set enumerated by the procedure of which e is the Gödel number. In describing the construction for each proof, we shall suppose that a clerk is carrying out the simultaneous enumeration of R 0, R 1, R 2, …, in such a way that at each step only a finite number of sets have begun to be enumerated and only a finite number of the members of any set have been listed. (One plan the clerk can follow is to turn his attention at Step a to the enumeration of Re where e+1 is the number of prime factors of a. Then each Re receives his attention infinitely often.) We shall denote by “Re a ” the set of numbers which, at or before Step a, the clerk has listed as members of Re . Obviously, all the Re a are finite sets, recursive uniformly in e and a. For any a we can determine effectively the highest e for which Re a is not empty, and for any a, e we can effectively find the highest member of Re a , just by scanning what the clerk has done by Step a. Additional notations will be introduced in the proofs to which they pertain.

(Received June 06 1958)