Which of the following statements is correct?
- sufficient and necessary conditions about regularity of a language exist but not discovered yet.
There's no sufficient and necessary condition about regularity of a language.
Pumping lemma is a necessary condition for non-regularity of a language.
- Pumping lemma is a sufficient condition for non-regularity of a language.
I know #(4) is correct and #(3) is false because "the converse of this statement is not true: a language that satisfies these conditions may still be non-regular", but what can be said about (1) and (2)?