Intersection is the easiest example to show directly. Finite-state automata are closed under intersection because we can always create a pairwise state representing the operation of both of the original automata, and accept a string only if both automata accept. This effectively runs both automata in parallel.
No. The intersection of an infinite set of regular languages is not necessarily even computable. The closure of regular languages under infinite intersection is, in fact, all languages. The language of “all strings except s” is trivially regular.
A subset X of S is said to be closed under these methods, if, when all input elements are in X, then all possible results are also in X. Sometimes, one may also say that X has the closure property. (it is the intersection of all closed subsets that contain Y).
Closure Properties of Regular Languages Given a set, a closure property of the set is an operation that when applied to members of the set always returns as its answer a member of that set. For example, the set of integers is closed under addition.
Regular Languages are closed under intersection, i.e., if L1 and L2 are regular then L1 ∩ L2 is also regular. L1 and L2 are regular • L1 ∪ L2 is regular • Hence, L1 ∩ L2 = L1 ∪ L2 is regular.
Closure under Union For any regular languages L and M, then L ∪ M is regular. Proof: Since L and M are regular, they have regular expressions, say: Let L = L(E) and M = L(F). Then L ∪ M = L(E + F) by the definition of the + operator.
Let L be a regular language, and M be an NFA that accepts it. Here, δR is δ with the direction of all the arcs reversed. Thus, it is proved that L is closed under reversal.
Regular Languages are closed under intersection, i.e., if L1 and L2 are regular then L1 ∩ L2 is also regular. L1 and L2 are regular • L1 ∪ L2 is regular • Hence, L1 ∩ L2 = L1 ∪ L2 is regular.