If is a class of recursively
enumerable sets, then the set of Gödel numbers
of functions whose domains belong to
is called its index set. If the
index set of
is a recursive set, then
either
is empty or
contains all recursively enumerable sets.
Rice's theorem is an important result for computer science because it sets up boundaries for research in that area. It basically states that only trivial properties of programs are algorithmically decidable.