Quiz6Soln

# Quiz6Soln - something along the lines of move to the end of...

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CSCI 2670 Intro. to Theory of Comp. Quiz 6 10/12/2005 Name: _______________________________ (1) (5 points) In class and in the text, we used the following Turing machine E to enumerate a language accepted by Turing machine M. First, we determined a way of listing all strings in Σ* - i.e., we let Σ* = { s 1 , s 2 , s 3 , … }: E = “Ignore the input. 1 For i = 1, 2, 3, … 2 Run M for i steps on each input s 1 , s 2 , …, s i 3 Whenever M accepts a string, print it” Why didn’t we use the following method, which seems more intuitive? E bad = “Ignore the input. 1 For i = 1, 2, 3, … 2 Run M on input s i 3 If M accepts, print s i Because M may not halt on some s i . In this case, E bad would not print out any string s j where j > i even if s j is in L(M).

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(2) (5 points) Let L = {0 n 1 m | n < m}. Describe a 2-tape Turing machine that decides L. You may describe the Turing machine at a high level (e.g., you may say
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Unformatted text preview: something along the lines of move to the end of the tape). M = “on input w 1. if w starts with a 1 2. move right until a symbol other than 1 is encountered 3. if the tape head points to ~ accept 4. else reject 5. else if the tape head does not start with a 0 reject 6. replace the 0 with X 7. move right past all 0’s and Y’s 8. if the tape head does not point to a 1 reject 9. replace the 1 with a Y 10. move left past all Y’s 11. if the tape head points to an X 12. move right once then move right past all Y’s 13. if the tape head does not point to a 1 reject 14. move right past all 1’s 15. if the tape head points to a ~ accept 16. else reject 17. else move left past all 0’s 18. move right 1 space 19. go to line 6”...
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