# Is my proof for the regularity of the language $A/B$ correct?

This problem is from Sipser's Theory of Computation 3rd Edition.

1.35 Prove that $$A/B = \{\omega \ | \ \omega x \in A \ \mathrm{for\ some \ } x\in B\}$$ is regular where $$A$$ is regular and $$B$$ is any language.

I want to verify that my proof is correct. It is as follows:

Let $$A$$ be a regular language. Then it has a DFA $$N = (Q, \Sigma, \delta, q_0, F)$$ such that $$L(N) = A$$. We will construct a DFA $$M = (Q, \Sigma, \delta, q_0, F^{\prime})$$ that recognizes $$A/B$$ and only $$A/B$$.
Let $$P = \{p_1, \dots, p_k\} \subseteq Q$$ be the set of states of $$N$$ that have a path to a final state of $$N$$. We will construct a DFA corresponding to each of these states.
Let $$N_i = (Q, \Sigma, \delta, p_i, F)$$ be the DFA identical to $$N$$ except the start state is $$p_i \in P$$. This means $$L(N_i)$$ contains strings $$x$$ where $$\omega x \in A$$ and $$\delta^*(\omega, q_0) = p_i$$. Now we determine $$F^{\prime}$$:

$$F^{\prime} = \{p_i \in P : \exists x\in L(N_i) \cap B\}$$

We will now prove that $$L(M) = A/B$$.

$$(L(M) \subseteq A/B)$$:
Let $$\omega \in L(M)$$. Then by construction we have: $$\delta^*(q_0, \omega) = f^{\prime} \in F^{\prime} \\ \Rightarrow f^{\prime} = p_i \in P \ \ \mathrm{and} \ \ \exists x\in B\cap L(N_i) \ \ \mathrm{such \ that} \ \delta^*(p_i, x) = f \in F \\ \Rightarrow \delta^*(q_0, \omega x) = f \in F \\ \Rightarrow \omega x \in A \ \ (\mathrm{we \ also \ know \ } x \in B) \\ \Rightarrow \omega \in A/B$$

$$(A/B \subseteq L(M))$$:
Let $$\omega \in A/B$$. Then $$\exists x \in B$$ such that $$\omega x \in A$$. Let $$\delta^*(q_0, \omega) = p_i$$. ($$p_i \in P$$ is unique because $$N$$ is a DFA.) We know $$\omega x \in A$$ so $$\delta^*(p_i, x) = f \in F$$. Therefore $$x \in L(N_i)$$. We already know $$x \in B$$ so $$x \in B \cap L(N_i)$$. Hence $$p_i \in F^{\prime}$$. We conclude that $$\omega \in L(M)$$.

We have constructed a DFA that accepts $$A/B$$ and only $$A/B$$. Therefore $$A/B$$ is regular.

• What are the final states in each of the copies of $N$? are they all the same final states? – nir shahar Apr 24 at 9:35
• @nirshahar Yes, the same final states as $N$ itself. – Zara Apr 24 at 15:25
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