I have a logical question that I can't quite crack:

Given two automata accepting the same language $L$, does one have to refine the other?

In other words, if $A_1$ and $A_2$ both accept $L$, with associated equivalence relations $R_{A_1}$ and $R_{A_2}$, does $R_{A_1}$ have to refine $R_{A_2}$, or vice versa?

I am leaning toward the answer yes because if we have a regular language $L$ which is accepted by an automaton $A$, we can show that the relation $R_A$ refines the relation $R_L$, meaning $R_A \sqsubseteq R_L$, which means that both $R_{A_1} \sqsubseteq R_L$ and $R_{A_2} \sqsubseteq R_L$.

We are currently studying the Myhill-Nerode Theorem, so I'm guessing it has something to do with it. I've tried combining few theorems together, but came out empty.


1 Answer 1


For an integer $n$, consider the following DFA $A_n$ on the alphabet $\{a\}$. The set of states is $\{q_0,\ldots,q_{n-1}\}$. The initial state is $q_0$. All states are accepting. The transition function is $\delta(q_i,a) = q_{i+1}$, where we identify $q_n$ with $q_0$.

Do the relations of $A_2$ and $A_3$ refine each other?

  • $\begingroup$ I appreciate the quick reply. Perhaps I was using the wrong word, instead of "accepting" I meant "implementing" the same language. Does this make a difference? $\endgroup$
    – Ben
    Apr 8, 2021 at 16:09
  • $\begingroup$ I don't think so. $\endgroup$ Apr 8, 2021 at 17:01
  • $\begingroup$ however, A2 does refine A3. R_1 refine the equivalence relation R_2 if for each x,y that belongs to A if x,y belongs to R1 then they belong to R2. in your example A2 represents the letters a,b. that means that A3 includes a,b,c . No? sorry this is a new subject so it's going to take me some time to understand it $\endgroup$
    – Ben
    Apr 8, 2021 at 17:35
  • 1
    $\begingroup$ All my automata are over the same alphabet, $\{a\}$. $\endgroup$ Apr 8, 2021 at 17:39

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