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Material Type: Assignment; Class: Discrete Structures II; Subject: Computer Theory; University: University of Central Florida; Term: Fall 2000;
Typology: Assignments
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1) (5 pts) Let a k-PDA be a pushdown automaton that has k stacks. Thus a 0-PDA is an NFA and a 1-PDA is a convensional PDA. You already know that 1-PDAs are more powerful (recognize a larger class of languages) than 0-PDAs. Show that a 2-PDA is more powerful than a 1-PDA. (Hint: Simulate a Turing machine tape with two stacks.) 2) A Turing machine with left reset is similar to an ordinary Turing machine except that the transition function has the form : Q x Q x x {R, RESET}. If (q,a) = (r,b,RESET), when the machine is in state q reading a, the machine’s head jumps to the left-hand end of the tape after it writes b in the tape and enters state r. Note that these machines do not have the usual ability to move the head one symbol left. Show that Turing machines with left reset recognize the class of Turing-recognizable languages(ie. they have the same power as a standard Turing machine.) 3) (6 pts) A Turing machine with stay put instead of left is similar to an ordinary Turing machine except that the transition function has the form : Q x Q x x {R, S}. At each point the machine can move its head right or let it stay in the same position. Show that this Turing machine variant is NOT equivalent to the usual version. (Bonus: What class of languages do these machines recognize?) 4) (3 pts each) Show that the collection of decidable languages is closed under the operations of a) union b) concatenation c) star d) complementation e) intersection 5) (2 pts) Let A be the language containing only the single string s, where s = 0, if God does not exists 1, if God exists Is A decidable? Why or why not? 6) (6 pts) Show that you can decide if two DFAs are equivalent(accept the exact same set of strings) or not. Give an algorithm that is guaranteed to terminate that correctly decides this problem. 7) (5 pts) Build a Turing machine that enumerates the set of even length strings over {a}. 8) (6 pts) Prove that there is no algorithm with input consisting of a Turing machine M = {Q, , , , q 0 , F), a state qi Q, and a string w *^ that determines whether the computation of M with input w enters state
9) (5 pts) Show that the language {aibj^ | i,j 0 i j} is not regular by using the the pumping lemma. 10) (5 pts) Show that the language {ak!^ | k > 0} is not context free by using the pumping lemma. (Note: Problems 1-6 come from Introduction to the Theory of Computation by Michael Sipser. The other two problems come from the text.)