i have a theoretical questions, and was wondering if you could help me with it so i could understand the material better.

1)suppose we have some language L over $\Sigma$, can we know if L is regular if all of it equivalence classes of relation $R_L$ is finite?

2)and what about L*? is it finite? i mean the cardinality is infinity(alef), but can it be made into a finite language?

Edit: my question is about Myhill–Nerode relation

i am curious about those and your input would really help me understand more about it

thank you very much!

  • $\begingroup$ How is $R_L$ defined? Are you referring to the Myhill-Nerode relation? $\endgroup$
    – dkaeae
    Dec 3, 2018 at 8:39
  • $\begingroup$ yes, thank you very much. i'll state boldly that this is the relation i was refering to $\endgroup$
    – mathnoobie
    Dec 3, 2018 at 8:45
  • $\begingroup$ I don't understand your second question. "Finite" means "finite cardinality". $\endgroup$ Dec 3, 2018 at 8:50

1 Answer 1


One of the main results of Myhill–Nerode theory is that a language is regular iff its Myhill–Nerode relation has finitely many equivalence classes. You can find a proof in many online resources. In particular, if all equivalence classes of the Myhill–Nerdoe relation of a language are finite, then it must have infinitely many equivalence classes, and so be non-regular.

The language $L^*$ is finite in two cases: if $L = \emptyset$ or if $L = \{\epsilon\}$. In any other case it is infinite.


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