tail (L) = {y : xy ∈ L for some x ∈ Σ∗} .Show that if L is regular, so is tail (L).
Question
tail (L) = {y : xy ∈ L for some x ∈ Σ∗} .Show that if L is regular, so is tail (L).
Solution
To prove that if L is regular, so is tail(L), we can follow these steps:
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Assume L is regular. By definition, a language is regular if there exists some finite automaton M that accepts it. Let's say M is the finite automaton that accepts L.
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We need to construct a finite automaton M' that accepts tail(L).
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The automaton M' will have the same states, transition function, and final states as M. However, the start state of M' will be different.
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In M', the start state will be a new state that has ε-transitions (transitions on the empty string) to every state in M. This is because for any string y to be in tail(L), there must exist some string x such that the concatenation xy is in L. By adding ε-transitions from the start state of M' to every state in M, we are effectively allowing for any string x to be "skipped" in the input.
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Therefore, M' accepts a string if and only if there exists some string x such that the concatenation xy is accepted by M. This is exactly the condition for a string to be in tail(L).
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Since we have constructed a finite automaton that accepts tail(L), we can conclude that tail(L) is regular.
This completes the proof.
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