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It is well known that regular languages are characterized by the Myhill-Nerode equivalence. For language $L$ over $\Sigma^*$ define the equivalence $x\sim_L y$ over $\Sigma^*$ iff for all $z\in\Sigma^*$ we have $xz\in L \iff yz\in L$. Then $L$ is regular iff $\sim_L$ is of finite index, i.e., has a finite number of equivalence classes.

I know that the relation can be used to show that some languages are not regular, by indicating infinitely many strings that are not equivalent.

My question: can we easily use Myhill-Nerode to show closure properties of regular languages? Or should we use the "syntactic congruence" of languages?

As an example for prefix it is easy, as $x\sim_L y$ implies $x\sim_{\mbox{pref}(L)} y$. But how do we handle suffix, concatenation, star, mirror?

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The equivalence classes of the Myhill-Nerode relation are also the states of the minimal DFA for the language. So whatever is easy to show using DFAs, you can convert to a proof which uses the Myhill-Nerode point of view. Wherever you need NFAs, you can expect the proof to get more complicated.

But the correct answer is: try it out for yourself! That's how research is done.

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Well, I was not asking for original research, thanks. I was hoping that someone knew whether the knowledge of $\sim_K$ being finite index would predict $\sim_{\phi(K)}$ being finite index, where $\phi$ is a language operation. I tried for $\phi$=prefix, but I cannot see how concatenation is handled in an elegant way. Yes, I can construct a decent DFA for it. – Hendrik Jan Dec 16 '12 at 21:15

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