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If $L\subset \Sigma^\ast$ is a regular language then it is known that we can find a directed graph (aka automaton) $G$ with edges labelled with symbols from $\Sigma$, an ‘initial’ vertex, and a set of ‘terminal’ vertices, with the property that a word $w$ is in $L$ iff $w$ is the sequence of edge labels along a path from the initial vertex to a terminal vertex.

It’s possible that the same word can appear as a result of different paths in $G$. Can we guarantee that for any regular $L$ we can find a $G$ such that each word in $L$ appears as a path in $G$ in a unique way?

(I’m interested in this because I want to know whether the solution to the problem I worked on here applies cleanly to any regular language, or just for a certain class of them.)

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  • $\begingroup$ This question doesn't look like a research-level question in TCS. It should perhaps be moved to cs.stackexchange.com $\endgroup$
    – J.-E. Pin
    Aug 29, 2013 at 10:23

3 Answers 3

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Maybe I misunderstood your question, but wouldn't a Deterministic Finite Automaton do what you want?

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  • $\begingroup$ You may be right. I'm not knowledgeable in this area and I've been thinking in terms of generation of strings, not recognition. But now I think about it, you're probably right. I'll mull over it for a little bit. $\endgroup$
    – user381
    Aug 23, 2010 at 21:47
  • $\begingroup$ Yes, it's straightforward. I just came at this at the wrong angle. Thanks. $\endgroup$
    – user381
    Aug 23, 2010 at 21:51
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You may want to know that there's also a notion of ambiguity for regular expressions. Also, you can decide ambiguity for NFAs in $O(n^2)$.

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A DFA, as stated, can answer your question.

If you are interested in a small automaton for this task there are three things which comes to mind:

  1. DFA can be optimized, so you could find the minimal (state-wise) automaton for the langauge.
  2. There exists a different type of automaton called Unambiguous Finite Automaton ($UFA$), which is a non deterministic finite automaton which has at most one accepting path for each word.
  3. Unlike DFA, UFA optimization is NP-hard hence the minimal automaton for your task is unlikely to be found efficiently for a general language.
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