Eine Menge formula_2 natürlicher Zahlen heiße "immun", falls gilt::formula_3Falls also formula_4 unendlich ist, aber keine unendliche rekursiv aufzählbare Teilmenge besitzt. ###>In recursion theory a subset of the natural numbers is called a simple set if it is co-infinite and recursively enumerable, but every infinite subset of its complement fails to be enumerated recursively.