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Unformatted text preview: Removing Nondeterminism from TwoWay Automata Giovanni Pighizzini Dipartimento di Informatica e Comunicazione Universit degli Studi di Milano Porto June 22, 2010 Outline The Question of Sakoda and Sipser Quasi Sweeping Automata and Quasi Sweeping Simulation Sadoka&Sipser Question vs L ? = NL Making Unary 2NFAs Unambiguous Conclusion Finite State Automata i n p u t ... 6 Base version: oneway deterministic finite automata (1DFA) I oneway input tape I deterministic transitions Possibile variants allowing: I nondeterministic transitions oneway nondeterministic finite automata (1NFA) I input head moving forth and back twoway deterministic finite automata (2DFA) twoway nondeterministic finite automata (2NFA) I alternation I ... TwoWay Automata: Technical Details i n p u t ... a 6 I Input surrounded by the endmarkers and a I Transition function : Q ( { , a} ) 2 Q { 1 , , + 1 } where 1 , , + 1 are the possible movements of the input head I w * accepted iff there is a computation with input tape w a from the initial state q , scanning the left endmarker reaching a final state 1DFA, 1NFA, 2DFA, 2NFA What about the power of these models? They share the same computational power, namely they characterize the class of regular languages , however... ...some of them are more succinct Example: L = ( a + b ) * a ( a + b ) n 1 I L is accepted by a 1NFA with n + 1 states q q 1 q 2 q 3 q n @ @ R a a , b a , b a , b a , b I The minimum 1DFA accepting L requires 2 n states I We can get a deterministic automaton for L with n + 2 states, which reverses the input head direction just one time I Hence L is accepted by a 1NFA and a 2DFA with approx. the same number of states a minimum 1DFA exponentially larger Example: L = ( a + b ) * a ( a + b ) n 1 a ( a + b ) * I L is accepted by a 1NFA with n + 2 states q q 1 q 2 q 3 q n q f @R a a , b a , b a , b a a , b a , b I The minimum 1DFA accepting L uses 3 2 n 1 + 1 states I Using head reversals the number of states becomes linear I Even in this case L is accepted by a 1NFA and a 2DFA with linearly related numbers of states a minimum 1DFA exponentially larger Example: L = ( a + b ) * a ( a + b ) n 1 a ( a + b ) * b b a b a a b a a a a n = 4 while input symbol 6 = a do move to the right move n squares to the right if input symbol = a then accept else move n 1 cells to the left repeat from the first step Exception: if input symbol = a then reject I This can be implemented by a 2DFA with O ( n ) states I By a different algorithm, L can be also accepted by a 2DFA with O ( n ) states which changes the direction of its input head only at the endmarkers Costs of the Optimal Simulations Between Automata 1DFA 1NFA 2DFA 2NFA @ @ @ @ @ @ @ R ?...
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This note was uploaded on 01/28/2012 for the course CS 220 taught by Professor Ibarra,o during the Winter '08 term at UCSB.
 Winter '08
 Ibarra,O

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