Regular languages are closed under reversal, meaning if L is a regular language, then its reversed language LR is also regular. This is proven by creating a new automaton that reverses the transitions of the original DFA. Thus, the reversed language is also accepted by a finite automaton, confirming its regularity.
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.
Regular languages are closed under complement, union, intersection, concatenation, Kleene star, reversal, homomorphism, and substitution.
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.
Regular languages are closed under concatenation - this is demonstrable by having the accepting state(s) of one language with an epsilon transition to the start state of the next language. If we consider the language L = {a^n | n >=0}, this language is regular (it is simply a).
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.
Intersection. Theorem If L1 and L2 are regular languages, then the new language L = L1 ∩ L2 is regular. Proof By De Morgan's law, L = L1 ∩ L2 = L1 ∪ L2. By the previous two theorems this language is regular.
Intersection. Theorem If L1 and L2 are regular languages, then the new language L = L1 ∩ L2 is regular. Proof By De Morgan's law, L = L1 ∩ L2 = L1 ∪ L2. By the previous two theorems this language is regular.
In class, we proved that the set of regular languages is closed under union. The idea behind the proof was that, given two DFAs D1,D2, we could make a new DFA D3 which simultaneously keeps track of which state we're at in each DFA when processing a string.