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Unformatted text preview: CS 154 Intro. to Automata and Complexity Theory Handout 27 Autumn 2009 David Dill October 27, 2009 Solution Set 5 Problem 1 Both recursive languages and recursively enumerable languages are closed under intersection. Any recursive language is accepted by a TM that is guaranteed to halt after a finite number of steps, whether or not the input string is in the language. Any recursively enumerable language has a TM that will eventually halt in an accepting state when run on any input that is in the language. To compute L 1 intersectiontext L 2 , we can construct a TM that first simulates the TM for L 1 , then simulates the TM for L 2 , and halts in an accepting state if and only if both simulated TMs halt in accepting states. If L 1 and L 2 are both recursive, then both simulations are guaranteed to halt after a finite number of steps, so the combined TM for L 1 intersectiontext L 2 also halts after a finite number of steps. Note that any input that is in L 1 intersectiontext L 2 is also in both languages L 1 and L 2 . If L 1 and L 2 are both recursively enumerable, then when run on an input that is in L 1 intersectiontext L 2 , both simulations will halt in an accepting state, meaning that the combined TM for L 1 intersectiontext L 2 will also halt in an accepting state. Problem 2 Let L denote the language described in the problem statement. The language L is undecidable, but it is recursively enumerable. To show that L is undecidable, we do a proof by contradiction using a reduction from L u . Suppose that L is decidable, meaning that there exists a TM M L that is an algorithm for L . Given an input ( M,w ) , we would like to decide if ( M,w ) L u , i.e. if the TM M encoded by the string ( M ) would accept the input w . To do so, we first construct a new machine M that uses tape alphabet = { , 1 , 2 ,B } and does the following: simulate the execution of machine M on input w (using only tape symbols 0, 1, and B to perform the simulation), and if the simulation halts in...
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This note was uploaded on 01/12/2010 for the course CS 154 at Stanford.
 '08
 Motwani,R

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