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I've faced this question in my homework and I couldn't provide elegant proof for it.

We're given $\Sigma_n=\{1,\dots,n\}$ and a language: $$L_n =\{w\in\Sigma^*_n\ |\ \exists\sigma\in\Sigma_n\ :\ ⋕_\sigma(w)=0\}$$ That is a language that consists of letters from $\Sigma_n$ but doesn't contain all the letters.

Question: We're asked to prove that there is no $DFA$ with less than $2^n$ states that accepts $L_n$.

Note: It's given that there is $NFA$ with $(n+1)$ states that accepts $L_n$.

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  • $\begingroup$ Have you learned "Myhill–Nerode theorem"? Have you read the proof for it? $\endgroup$
    – John L.
    Mar 18, 2022 at 20:26
  • $\begingroup$ Yea I did, I tried actually to use it but it was hard at some point. $\endgroup$
    – Mohamad S.
    Mar 18, 2022 at 20:28
  • $\begingroup$ Suppose $n=1$. List all different equivalence classes, where each class can be represented by a (shortest) string in it. Do the same for $n=2$. You may see the pattern. $\endgroup$
    – John L.
    Mar 18, 2022 at 20:33
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    $\begingroup$ @JohnL. Thanks John, I appreciate your tip, I will try to follow it and understand how the proof works. $\endgroup$
    – Mohamad S.
    Mar 18, 2022 at 20:41

1 Answer 1

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You can use the Myhill-Nerode theorem for the given task. You must provide at least $2^n$ prefixes $p_i\in \Sigma_n^*$ belonging to the different equivalence classes, e.g. s.t. for every two prefixes $p_i$, $p_j$ there exists a suffix $s_{i,j}$ s.t. $p_i s_{i,j}\in L_n$ and $p_j s_{i,j}\notin L_n$ or vice versa.

Given your task, such prefixes correspond to all possible subsets of the alphabet $\Sigma_n$. There are $2^n$ such subsets, and the strings containing all the letters from the chosen subset $S_i$ and no other letters definitely satisfy the Myhill-Nerode equivalence class criterion. Given two words $p_i$, $p_j$ corresponding to the subsets $S_i$ and $S_j$, s.t. $S_i\not\subseteq S_j$, the suffix $p_k$ corresponding to the set $S_k=\Sigma_n\setminus S_i$ discerns $p_i$ and $p_j$, since $p_i p_k\not\in L_n$ and $p_j p_k\in L_n$.

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  • $\begingroup$ I reviewed your answer and I have a question. What about the case where we're given two words $p_i ,\ p_j$ corresponding to the subsets $S_i$ and $S_j$, s.t. $S_i \subseteq S_j$? $\endgroup$
    – Mohamad S.
    Mar 21, 2022 at 16:36
  • $\begingroup$ If the subsets $S_i$ and $S_j$ are not equal, then $S_i\subseteq S_j$ simply implies $S_j\not\subseteq S_i$, and the symmetric reasoning holds. $\endgroup$
    – Tonita
    Mar 21, 2022 at 18:24
  • $\begingroup$ Oh right, thanks for the explanation. $\endgroup$
    – Mohamad S.
    Mar 21, 2022 at 19:38

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