s p vimal, department of csis, bits, pilani
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S P Vimal, Department of CSIS, BITS, Pilani. Agenda. 2. A quick review of syntax & semantics Entailment in Propositional Logic Discussion. EA C461 Artificial Intelligence. Review: Propositional logic: Syntax. 3. Propositional logic is the simplest logic - PowerPoint PPT PresentationTRANSCRIPT
S P Vimal, Department of CSIS, BITS, Pilani
Agenda
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A quick review of syntax & semantics Entailment in Propositional LogicDiscussion
Review: Propositional logic: Syntax
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Propositional logic is the simplest logic
Atomic sentence : A Propositional symbol (written in upper case, just for notational convenience)
W1,3 – Wumpus is in room (1,3) True / False
Review: Propositional logic: Syntax
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If S is a sentence, S is a sentence (negation) If S1 and S2 are sentences, (S1 S2)is a sentence
(conjunction) If S1 and S2 are sentences, ( S1 S2) is a sentence
(disjunction) If S1 and S2 are sentences, (S1 S2) is a sentence
(implication) If S1 and S2 are sentences, (S1 S2) is a sentence
(biconditional)Precedence , , , ,
Semantics
Defines the rules for defining the truth of a sentence w.r.t a particular model.
Let the sentence in the KB make use of the proposition symbols P12,P22,P31. One possible model is m1 =
{P12=true,P22=false,P31=true}
With these symbols, 8 possible models, can be enumerated automatically.
Rules for evaluating truth with respect to a model m:S is true iff S is false S1 S2 is true iff S1 is true and S2 is trueS1 S2 is true iff S1is true or S2 is trueS1 S2 is true iff S1 is false or S2 is true i.e., is false iff S1 is true and S2 is falseS1 S2 is true iff S1S2 is true and S2S1 is true
Review: Truth tables for connectives
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Wumpus world sentences
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Assume a wumpus world without wumpus (??)
Let Pi,j be true if there is a pit in [i, j].
Let Bi,j be true if there is a breeze in [i, j].
R1: P1,1
"Pits cause breezes in adjacent squares"
R2: B1,1 (P1,2 P2,1)
R3: B2,1 (P1,1 P2,2 P3,1)
R4: B1,1R5: B2,1
Truth tables for inference
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Inference by enumeration
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Depth-first enumeration of all models is sound and complete
For n symbols, time complexity is O(2n), space complexity is O(n)
Equivalence
Two sentences α and β are equivalent, if they are true in the same set of models, which is written as α <=> β
In other words, α ≡ β if and only if α ╞ β and β ╞ α
Validity
A sentence is valid, if it is true in all models Example : P V ר P Tautologies
Deduction theorem:
For any sentence α and β, α ╞ β if and only if (α ╞ β ) valid
Now, What is inference? Rephrase in terms of Deduction Theorem? All you do is to check the validity of KB╞ α
Conversely???
Conversely, every valid implication represents an inference.
Satisfiablity
A sentence is satisfiable if it is true in some model.
If a sentence α is true in model m, then m satisfies α, or we say m is the model of α.
How do you verify if a sentence is satisfiable? Enumerate the possible models, until one is found
to satisfy.
Many problems in Computer Science can be reduced to a satisfiablity problems Ex. CSP is all about verifying if the constraints are
satisfiable by some assignments
Connection between Validity and Satisfiablity
α is valid iff רα is not satisfiable How do we use this observation for
‘entailment’?Let us phrase it this way
α╞ β if and only if the sentence (α Λ רβ ) is not satisfiable.
Reduction to an absurd thing / Proof by refutation / Proof by contradiction
Proof methods
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Proof methods divide into (roughly) two kinds: Application of inference rules
Legitimate (sound) generation of new sentences from old Proof = a sequence of inference rule applications
Can use inference rules as operators in a standard search algorithm
Typically require transformation of sentences into a normal form
Model checking truth table enumeration (always exponential in n) improved backtracking, e.g., Davis--Putnam-Logemann-
Loveland (DPLL) heuristic search in model space (sound but incomplete)
e.g., min-conflicts-like hill-climbing algorithms
A property
Monotonicity Set of entailed sentences can only increase as
information is added to the knowledge base. If KB β╞ then KB Λ α β ╞
Inference rules can be applied whenever suitable premises are found in the Knowledge base.
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Resolution
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Conjunctive Normal Form (CNF) conjunction of disjunctions of literals
clausesE.g., (A B) (B C D)
Resolution inference rule (for CNF):
li … lk, m1 … mn
li … li-1 li+1 … lk m1 … mj-1 mj+1 ... mn
where li and mj are complementary literals.
E.g., P1,3 P2,2, P2,2
P1,3
Resolution is sound and complete for propositional logic
Resolution algorithm
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Proof by contradiction, i.e., show KBα unsatisfiable
Resolution example
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KB = (B1,1 (P1,2 P2,1)) B1,1
α = P1,2
Completeness of Resolution
Resolution Closure (RC) RC(S) – set of clauses derivable by repeated
application of resolution rule to clauses in S or their derivatives
RC(S) must be finiteCompleteness is entailed by “Ground Resolution
Theorem” “If a set of clauses is unsatisfiable, then the
resolution closure of those clauses contains the empty clause ”
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Any complete search algorithm, applying only resolution can derive any conclusion entailed by any knowledge base in propositional logic
This is true only in restricted sense Resolution can always be used to either
confirm or refute a sentence Resolution cannot be used to enumerate
entailed sentences
Refutation completeness
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Contd…
Forward and backward chaining
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Horn Form (restricted)KB = conjunction of Horn clauses
Horn clause = proposition symbol; or (conjunction of symbols) symbol
E.g., C (B A) (C D B) Modus Ponens (for Horn Form): complete for Horn KBs
α1, … ,αn, α1 … αn ββ
Can be used with forward chaining or backward chaining.
These algorithms are very natural and run in linear time
Forward chaining
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Idea: fire any rule whose premises are satisfied in the KB, add its conclusion to the KB, until query is found
Forward chaining algorithm
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Forward chaining is sound and complete for Horn KB
Forward chaining example
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Forward chaining example
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Forward chaining example
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Forward chaining example
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Forward chaining example
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Forward chaining example
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Forward chaining example
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Forward chaining example
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Proof of completeness
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FC derives every atomic sentence that is entailed by KB1. FC reaches a fixed point where no new atomic
sentences are derived2. Consider the final state as a model m, assigning
true/false to symbols3. Every clause in the original KB is true in m
a1 … ak b
4. Hence m is a model of KB5. If KB╞ q, q is true in every model of KB, including m
Backward chaining
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Idea: work backwards from the query q:
to prove q by BC,check if q is known already, or
prove by BC all premises of some rule concluding q
Avoid loops: check if new subgoal is already on the goal stack
Avoid repeated work: check if new subgoal
1. has already been proved true, or
2. has already failed
Backward chaining example
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Backward chaining example
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Backward chaining example
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Backward chaining example
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Backward chaining example
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Backward chaining example
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Backward chaining example
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Backward chaining example
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Backward chaining example
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Backward chaining example
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Forward vs. backward chaining
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FC is data-driven, automatic, unconscious processing, e.g., object recognition, routine decisions
May do lots of work that is irrelevant to the goal
BC is goal-driven, appropriate for problem-solving, e.g., Where are my keys? How do I get into a PhD program?
Complexity of BC can be much less than linear in size of KB
Refining model checking
Algorithms checks satisfiability The connection between finding a solution to a
CSP and finding satisfying model for a logical sentence implies the existence of backtracking algorithm
DPLL (Davis, Putnam, Logemann & Loveland)
The DPLL algorithm
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Determine if an input propositional logic sentence (in CNF) is satisfiable.
Improvements over truth table enumeration:1. Early termination
A clause is true if any literal is true.A sentence is false if any clause is false.
2. Pure symbol heuristicPure symbol: always appears with the same "sign" in all clauses. e.g., In the three clauses (A B), (B C), (C A), A and B are
pure, C is impure. Make a pure symbol literal true.
3. Unit clause heuristicUnit clause: only one literal in the clauseThe only literal in a unit clause must be true.
The DPLL algorithm
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The WalkSAT algorithm
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Incomplete, local search algorithm Evaluation function: The min-conflict heuristic of minimizing the
number of unsatisfied clauses Balance between greediness and randomness
The WalkSAT algorithm
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Hard satisfiability problems
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Consider randomly generated 3-CNF sentences. e.g.,
(D B C) (B A C) (C B E) (E D B) (B E C)
m = number of clauses
n = number of symbols This CNF has 16 models out of 32 possible assignments. Quite easy to solve. So where are hard problems?
Fix n and keep increasing m,
Hard satisfiability problems
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Hard satisfiability problems
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Median runtime for 100 satisfiable random 3-CNF sentences
Inference-based agents in the Wumpus world
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A wumpus-world agent using propositional logic:
P1,1
W1,1
Bx,y (Px,y+1 Px,y-1 Px+1,y Px-1,y)
Sx,y (Wx,y+1 Wx,y-1 Wx+1,y Wx-1,y)
W1,1 W1,2 … W4,4
W1,1 W1,2
W1,1 W1,3 …
64 distinct proposition symbols, 155 sentences
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Keeping Track of Location and Orientation
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Should our KB include statements like
L11 Facing Right Forward L 21
What is the problem here?
L111Facing Right1 Forward1 L 21
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Facing Right TurnLeft 1 FacingUp2
Result : tens of thousands of such sentences
Summary
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Logical agents apply inference to a knowledge base to derive new information and make decisions
Basic concepts of logic: syntax: formal structure of sentences semantics: truth of sentences wrt models entailment: necessary truth of one sentence given another inference: deriving sentences from other sentences soundness: derivations produce only entailed sentences completeness: derivations can produce all entailed sentences
Wumpus world requires the ability to represent partial and negated information, reason by cases, etc.
Resolution is complete for propositional logicForward, backward chaining are linear-time, complete for Horn clauses
Propositional logic lacks expressive power