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Conjunctive Grammars and Alternating Automata Tamar Aizikowitz and Michael Kaminski Technion – Israel Institute of Technology WoLLIC 2008 Heriot-Watt University

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2 of 25 Introduction: ND Computational Models Non-deterministic computational models have existential acceptance conditions At least one computation must accept E.g., FSA, PDA, CFG, TM Languages accepted have disjunctive quality A word must meet one of many possible conditions (computations) Language class closed under union

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3 of 25 Introduction: Co-ND Computational Models Dual computational models have universal acceptance conditions All computations must accept E.g., Universal TM ; accepts class Co-NP Languages accepted have conjunctive quality A word must meet all conditions (computations) Language class closed under intersection

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4 of 25 Introduction: Combined Computational Models Several models combining existential and universal computations have been explored. We explore extensions of models for Context Free Languages. Specifically: Conjunctive Grammars Alternating Pushdown Automata

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5 of 25 Introduction: Conjunctive Grammars ( CG ) Introduced by Alexander Okhotin in 2001 * Extension of Context Free Grammars Add power of explicit intersection CG- s generate a larger class of languages * Okhotin A., Conjunctive Grammars, Journal of Automata, Languages and Combinatorics 6(4) (2001) 519-535 Reminder: Context free languages are not closed under intersection…

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6 of 25 Polynomial TimeCG LanguagesFinite ∩ CF Introduction: Conjunctive Grammars ( CG ) Conjunctive Grammars generate: Context Free languages Finite ∩ of CF languages Some more languages Generated languages are polynomial No known non-trivial technique to prove a language cannot be generated by a CG Exact placing in the Chomsky Hierarchy not known Context Free

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7 of 25 Introduction: Alternating Automata Alternating Automata were introduced by Chandra et.al. in 1981 * Computations alternate between existential and universal acceptance modes Well known examples: Alternating Finite State Automata (Verification) Alternating Turing Machines (Complexity Theory) Both equivalent to non-alternating counterparts * Chandra, A.K., Kozen, D.C., Stockmeyer, L.J., Alternation. Journal of the ACM 28(1) (1981) 114-133

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8 of 25 Introduction: Alternating Pushdown Automata ( APDA ) Further explored by Ladner et.al. in 1984 * Add conjunction to computations Not equivalent to standard PDA model Accept exactly the Exp. Time Languages Not equivalent to the CG model * Ladner, R.E., Lipton, R.J., Stockmeyer, L.J., Alternating pushdown and stack automata. SIAM Journal on Computing 13(1) (1984) 135-155

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9 of 25 Introduction: Synchronized APDA ( SAPDA ) We introduce a new model: Synchronized Alternating Pushdown Automata Equivalent to Conjunctive Grammar model In fact, this is the first class of automata suggested for Conjunctive Grammars

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10 of 25 Outline Conjunctive Grammars Synchronized Alternating PDA Equivalence Results: CG ~ SAPDA Future Work

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11 of 25 Conjunctive Grammars: Model Definition G = ( V, T, P, S ) V,T,S as in the standard CFG case P contains rules X → ( 1 & & n ) n = 1 gives standard CFG rules Conjunctive Formulas: { } V T are formulas If and are formulas then is a formula If 1,…, n are formulas then ( 1 & & n ) is a formula conjunct

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12 of 25 Conjunctive Grammars: Model Definition Derivation: Application: s 1 X s 2 s 1 ( 1 & & n )s 2 s.t. X → ( 1 & & n ) P Contraction: s 1 (w & & w) s 2 s 1 w s 2 Language: L(G) = {w T * | S * w} Note: ( & ) * w iff * w and * w

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13 of 25 Conjunctive Grammars: Example 1 L = {a n b n c n | n } Note: L = L 1 L 2 where L 1 = {a n b n c i | n,i } L 2 = {a i b n c n | n,i } G = ({S,S 1,S 2,S 3,S 4 }, {a,b,c}, S, P) where P = : S 1 → a S 1 b | ;S 2 → c S 2 | S 3 → a S 3 | ;S 4 → b S 4 c | S → (S 1 S 2 & S 3 S 4 ) S1S2*L1S1S2*L1 S3S4*L2S3S4*L2 L1 L2L1 L2

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14 of 25 Conjunctive Grammars: Example 1 Derivation of aaabbbccc : S (S 1 S 2 & S 3 S 4 ) (aS 1 bS 2 & S 3 S 4 ) … (aaaS 1 bbbS 2 & S 3 S 4 ) (aaabbbS 2 & S 3 S 4 ) (aaabbbcS 2 & S 3 S 4 ) … (aaabbbccc & S 3 S 4 ) … (aaabbbccc & aaabbbccc) aaabbbccc S → (S 1 S 2 & S 3 S 4 )S 1 → aS 1 b S 1 → S 2 → cS 2 contraction S 2 → cS 2

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Conjunctive Grammars: Interesting Languages CG s can generate some interesting languages: Multiple agreement: {a n b n c n | n } Cross agreement: {a n b m c n d m | n,m } Reduplication: {w$w | w {a,b} * } all mildly context-sensitive languages ( MCS ) CG s can also generate “stronger” languages such as: {ba 2 ba 4 ba 2 n b | n } not MCS because not semi-linear! 15 of 25

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16 of 25 Synchronized Alternating Pushdown Automata ( SAPDA ) Extension of the standard PDA model Transitions are to conjunctions of (state,stack-word) pairs E.g. (q, , X ) = {( p 1, XX ) ( p 2, Y ), ( p 3, Z ) } If all conjunctions are of one pair then we have the standard PDA model Non-deterministic model = many possible conjunctions

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Synchronized Alternating Pushdown Automata ( SAPDA ) Stack memory is a tree Each leaf has a separate processing head A conjunctive transition to n pairs splits the current branch to n branches Branches are processed independently Sibling branches must empty synchronously. A B C D A q p 17 of 25

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18 of 25 SAPDA: Model Definition A = (Q, , , , q 0, ) For every (q, , X ) Q ( { }) (q, , X ) {(q 1, 1 ) (q n, n ) | q i Q, i *, n } Example: (q, , X ) = {( p 1, Z ) ( p 2, YY )} Y Y Z p1p1 p2p2 X q

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Configuration: a labeled tree T where Internal nodes labeled α denoting stack contents Leaves labeled (q,w,α) denoting current state, remaining input and stack contents Initial Configuration: the tree T 0 which is the tree Accepting Configuration: a tree T e s.t. it is the tree for some q Q 19 of 25 SAPDA: Configurations (q0,w,)(q0,w,) (q,, )(q,, ) BA (q,ba, A)( p,a, DC ) A B C D A q p abba

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20 of 25 SAPDA: Computation Computation: Each computation step, a transition is applied to one stack-branch If a stack-branch empties, it cannot be selected If all siblings branches are empty and “synchronized” then they are collapsed Synchronized Collapsing: All siblings… Are empty Have the same state Have the same remaining input Are all labeled (q,w, ɛ )

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21 of 25 SAPDA: Language Accepting Computation: The final configuration is an accepting one, i.e. (q, , ) for some q Q. L(A) = {w * | A has an accepting computation on w} Note: Acceptance by accepting states can also be defined. Both models of acceptance are equivalent.

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SAPDA: Example L = {w {a,b,c} * | # a (w) = # b (w) = # c (w)} 22 of 25 a a b c c a b b c δ (q 0, ε, ) = (q 1, 1 ) (q 2, 2 ) δ (q 1, a, 1 ) = (q 1, a 1 )δ (q 2, a, 2 ) = (q 2, 2 ) δ (q 1, a, a) = (q 1, aa) δ (q 2, a, 2 ) = (q 2, 2 ) δ (q 1, b, a) = (q 1, ε) δ (q 2, b, 2 ) = (q 2, b 2 ) δ (q 1, c, a) = (q 1, a)δ (q 2, c, b) = (q 2, ε) δ (q 1, ε, 1 ) = (q 0, ε)δ (q 2, ε, 2 ) = (q 0, ε) q1q1 q2q2 q0q0 q0q0 q0q0 ε 11 22 a a b ε ε

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23 of 25 Equivalence Results Theorem 1: SAPDA CG Theorem 2: CG 1-state SAPDA ( 1SAPDA ) Corollary 1: SAPDA ~ CG Corollary 2: SAPDA ~ 1SAPDA Proofs: Both are extensions of the classical ones Surprisingly, the grammar-to-automaton translation is the more complicated one…

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Future Directions Linear CG and 1-turn SAPDA It is well known that LCFG ~ 1-turn PDA Linear CG : one non-terminal in each conjunct 1-turn SAPDA : each stack-branch turns once Initial results point towards: 1-turn SAPDA ~ LCG Finite-turn SAPDA, Deterministic SAPDA … Possibly, find a method to prove a language cannot be accepted by an SAPDA / CG … 24 of 25

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