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Unformatted text preview: accepted of length between n and 2n; that for NFAs, there is an NFA that is not universal, but the shortest string rejected is exponential in the number of states. We discussed the representation of FAs as labeled directed graphs and DFS and BFS as techniques for answering these questions. I also proved that a set over a singleletter alphabet is regular iff it is ultimately periodic. I also mentioned (but did not prove) that over any alphabet, the number of strings of each length in a regular set satisfy a linear recurrence. There was a little time left at the end, so I showed them that the set of first halves of strings in a regular set is regular. Monday Oct 5 definition of pushdown automata, acceptance by finalstate and by empty store equivalence of acceptance by final state and empty store acceptance of contextfree language by pushdown automaton...
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 Fall '07
 HOPCROFT

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