Set is recursive (decidable) iff both it + complement are r.e. Halting-problem r.e. but not recursive. Friedberg 1958 enumerator-without-repetition. Foundation of complexity classes.
Set is recursive (decidable) iff both it + complement are r.e. Halting-problem r.e. but not recursive. Friedberg 1958 enumerator-without-repetition. Foundation of complexity classes.