# An example of a decidable language that is not a CFL Implementation-level description of a TM State diagram of TM

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1 Turing Machines Review An example of a decidable language that is not a CFL Implementation-level description of a TM State diagram of TM Varieties of TMs Multi-Tape TMs Nondeterministic TMs String Enumerators 1 Turing Machines Blank part of tape Just like a DFA except: You have an infinite tape memory (or scratchpad) on which you receive your input and on which you can do your calculations You can read one symbol at a time from a cell on the tape, write one symbol, then move the read/write pointer (head) left (L) or right (R) 2

2 Who s Turing? Alan Turing ( ): one of the most brilliant mathematicians of the 20 th century (one of the founding fathers of computing) Click on Theory Hall of Fame link on class web under Lectures Introduced the Turing machine as a formal model of what it means to compute and solve a problem (i.e. an algorithm ) Paper: On computable numbers, with an application to the Entscheidungsproblem, Proc. London Math. Soc. 42 (1936). 3 How do Turing Machines compute? δ(current state, symbol under the head) = (next state, symbol to write over current symbol, direction of head movement) Diagram shows: δ(q 1,1) = (q rej, 0, L) (R = right, L = left) In terms of Configurations : 110q q rej 000 4

3 Solving Problems with Turing Machines We know L = {0 n 1 n 0 n n 0} is not a CFL (pumping lemma) Show L is decidable Construct a decider M such that L(M) = L A decider is a TM that always halts (in q acc or q rej ) and is guaranteed not to go into an infinite loop for any input 5 Idea for a Decider for {0 n 1 n 0 n n 0} General Idea: Match each 0 with a 1 and a 0 following the 1. Implementation Level Description of a Decider for L: On input w: 1. If first symbol = blank, ACCEPT 2. If first symbol = 1, REJECT 3. If first symbol = 0, Write a blank to mark left end of tape a. If current symbol is 0 or X, skip until it is 1. REJECT if blank. b. Write X over 1. Skip 1 s/x s until you see 0. REJECT if blank. c. Write X over 0. Move back to left end of tape. 4. At left end: Skip X s until: a. You see 0: Write X over 0 and GOTO 3a b. You see 1: REJECT c. You see a blank space: ACCEPT 6

4 State Diagram 0 R q 0 _,R 0 q 1 X,R skip0 q 0 X,L skip1 q go-left X L 0 L 1 L 0 X,R Try running the decider on: 010, , ACCEPT 0, 000, 0100, REJECT What about ? q REJ q ACC q at-left 7 Houston, we have a problem with our Turing machine 8

5 What s the problem? 0 R q 0 _,R 0 q 1 X,R skip0 q 0 X,L skip1 q go-left X L 0 L 1 L 0 X,R q REJ q ACC The decider accepts incorrect strings: , ACCEPT!!! q at-left Maybe it s the GOTO? Accepts (0 n 1 n 0 n ) * Need to fix it E. W. Dijkstra 9 An Aside: Dijsktra on GOTOs For a number of years I have been familiar with the observation that the quality of programmers is a decreasing function of the density of go to statements in the programs they produce. Opening sentence of: Go To Statement Considered Harmful by Edsger W. Dijkstra, Letter to the Editor, Communications of the ACM, Vol. 11, No. 3, March 1968, pp

6 A Simple Fix (to the Decider) Scan initially to make sure string is of the form 0*1*0* On input w: 1. If first symbol = blank, ACCEPT Add this 2. If first symbol = 1, REJECT 3. If first symbol = 0: if w is not in 00*11*00*, REJECT; else, Write a blank to mark left end of tape a. If current symbol is 0 or X, skip until it is 1. REJECT if blank. b. Write X over 1. Skip 1 s/x s until you see 0. REJECT if blank. c. Write X over 0. Move back to left end of tape. 4. At left end: Skip X s until: a. You see 0: Write X over 0 and GOTO 3a b. You see 1: REJECT c. You see a blank space: ACCEPT GOTO okay here 11 The Decider TM for L in all its glory q 0 q REJ 0 _,R 0 R 0 R 0 R q 1 q2 0 R q3 0 L 1 L _ L q REW q REJ q 1 X,R skip0 q 0 X,L skip1 q go-left New part tests for 00*11*00* X L 0 L 1 L q ACC 0 X,R q at-left 12

7 Varieties of TMs What if we allow multiple tapes? What if we allow nondeterminism? What if I take nap? 13 Various Types of TMs Multi-Tape TMs: TM with k tapes and k heads δ: Q Γ k Q Γ k {L,R} k δ(q i, a 1,, a k ) = (q j, b 1,, b k, L, R,, L) Nondeterministic TMs (NTMs) δ: Q Γ Pow(Q Γ {L,R}) δ(q i, a) = {(q 1, b, R), (q 2, c, L),, (q m, d, R)} Enumerator TM for L: Prints all strings in L (in any order, possibly with repetitions) and only the strings in L Other types: TM with Two-way infinite tape, TM with multiple heads on a single tape, 2D infinite tape TM, Random Access Memory (RAM) TM, etc. 14

8 Surprise! All TMs are born equal Each of the preceding TMs is equivalent to the standard TM They recognize the same set of languages (the Turingrecognizable languages) Proof idea: Simulate the deviant TM using a standard TM Example 1: Multi-tape TM on a standard TM Represent k tapes sequentially on 1 tape using separators # Use new symbol a to denote a head currently on symbol a b a h # 0 1 # b a h # # (See text for details) 15 Example 2: Simulating Nondeterminism Any nondeterministic TM N can be simulated by a deterministic TM M NTMs: δ: Q Γ Pow(Q Γ {L,R}) No ε transitions but can simulate them by reading and writing same symbol N accepts w iff there is at least 1 path in N s tree for w ending in q ACC General proof idea: Simulate each branch sequentially Proof idea 1: Use depth first search? No, might go deep into an infinite branch and never explore other branches! Proof idea 2: Use breadth first search Explore all branches at depth n before n+1 a q 0 (q 1, b, R) (q 2, c, L) a b q REJ a q ACC This branch does not halt 16

9 Simulating Nondeterminism: Details, Details Use a 3-tape DTM M for breadthfirst traversal of N s tree on w: Tape 1 keeps the input string w Tape 2 stores N s tape during simulation along 1 path (given by tape 3) up to a particular depth, starting with w Tape 3 stores current path number E.g. ε = root node q = path made up of 3 rd child of 1 st child of 2 nd child of root See text for more details a q 0 (q 1, b, R) (q 2, c, L) q REJ Does not halt a 213 q ACC 17 Next Class: We ll rap up Chapta 3 and cova undecidability Can t wait for undecidability! That guy needs help Have a great weekend! 18

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