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I'm reading Computer Science: An Interdisciplinary Approach. It mentioned an NFA (nondeterministic finite automaton) with a null transition, as shown above, recognizes all binary strings that do not contain bba as a substring.

I didn't quite understand why it says so. If the null transition happens before reading the bba, it stuck at state 2 and will report yes at the end?

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If a run of an automaton reaches a state where it has no legal transitions for the next input character, that run rejects immediately.

So, in your example, if the automaton makes the null transition, it can read $bb$ while staying in state $2$. But when it reads the $a$, it has no legal transition so that run rejects. Any run that stays in states $0$ and $1$ must also reject, because those two states are rejecting states. Therefore, the automaton does indeed reject all strings containing $bba$.

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  • $\begingroup$ Thanks @david-recherby "If a run of an automaton reaches a state where it has no legal transitions for the next input character, that run rejects" So it can reject the whole input without needing to reach the end, right? $\endgroup$
    – Qiang Huang
    Sep 28 '18 at 16:48
  • $\begingroup$ That's correct. (I added the word "immediately" to the answer to suggest this more strongly.) $\endgroup$
    – David Richerby
    Sep 28 '18 at 16:52
  • $\begingroup$ safaribooksonline.com/library/view/computer-science-an/… In the above example, the author says it will accept ababb because we can establish the sequence of state transitions as 0-0-1-0-2-2-2. But this is not deterministic, it is only one possible result, right? $\endgroup$
    – Qiang Huang
    Sep 28 '18 at 17:03
  • $\begingroup$ Right. The automaton is nondeterministic, so it accepts as long as there is one or more accepting run. $\endgroup$
    – David Richerby
    Sep 28 '18 at 17:04

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