logic in ai - university of washington...truth, and often nothing so strange. 7 truth •francis...
TRANSCRIPT
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Logic in AI Chapter 7
Mausam
(Based on slides of Dan Weld, Stuart Russell, Subbarao Kambhampati, Dieter Fox,
Henry Kautz…)
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Knowledge Representation
• represent knowledge about the world in a manner that facilitates inferencing (i.e. drawing conclusions) from knowledge.
• Example: Arithmetic logic – x >= 5
• In AI: typically based on
– Logic
– Probability
– Logic and Probability
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Common KR Languages
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Prop logic
First order predicate logic
(FOPC)
Prob. Prop. logic
Objects,
relationsDegree of
belief
First order Prob. logic
Objects,
relations
Degree of
belief
Degree of
truth
Fuzzy Logic
Time
First order Temporal logic
(FOPC)
Epistemological commitment
Ontological commitment
t/f/u Deg belief
facts
Facts Objects relations
Prop logic
Prob prop logic
FOPC Prob FOPC
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KR Languages
• Propositional Logic
• Predicate Calculus
• Frame Systems
• Rules with Certainty Factors
• Bayesian Belief Networks
• Influence Diagrams
• Ontologies
• Semantic Networks
• Concept Description Languages
• Non-monotonic Logic
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Basic Idea of Logic
• By starting with true assumptions, you can deduce true conclusions.
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Truth •Francis Bacon (1561-1626) No pleasure is comparable to the standing upon the vantage-ground of truth. •Thomas Henry Huxley (1825-1895) Irrationally held truths may be more harmful than reasoned errors. •John Keats (1795-1821) Beauty is truth, truth beauty; that is all ye know on earth, and all ye need to know.
•Blaise Pascal (1623-1662) We know the truth, not only by the reason, but also by the heart. •François Rabelais (c. 1490-1553) Speak the truth and shame the Devil. •Daniel Webster (1782-1852) There is nothing so powerful as truth, and often nothing so strange.
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Truth •Francis Bacon (1561-1626) No pleasure is comparable to the standing upon the vantage-ground of truth. •Thomas Henry Huxley (1825-1895) Irrationally held truths may be more harmful than reasoned errors. •John Keats (1795-1821) Beauty is truth, truth beauty; that is all ye know on earth, and all ye need to know.
•Blaise Pascal (1623-1662) We know the truth, not only by the reason, but also by the heart. •François Rabelais (c. 1490-1553) Speak the truth and shame the Devil. •Daniel Webster (1782-1852) There is nothing so powerful as truth, and often nothing so strange.
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Components of KR
• Syntax: defines the sentences in the language
• Semantics: defines the “meaning” to sentences
• Inference Procedure
– Algorithm
– Sound?
– Complete?
– Complexity
• Knowledge Base
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Knowledge bases
• Knowledge base = set of sentences in a formal language
• Declarative approach to building an agent (or other system): – Tell it what it needs to know
• Then it can Ask itself what to do - answers should follow from the KB
• Agents can be viewed at the knowledge level
i.e., what they know, regardless of how implemented
• Or at the implementation level i.e., data structures in KB and algorithms that manipulate them
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Propositional Logic • Syntax
– Atomic sentences: P, Q, …
– Connectives: , , ,
• Semantics – Truth Tables
• Inference – Modus Ponens – Resolution – DPLL – GSAT
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Propositional Logic: Syntax • Atoms
–P, Q, R, … • Literals
–P, P • Sentences
–Any literal is a sentence – If S is a sentence
• Then (S S) is a sentence • Then (S S) is a sentence
• Conveniences P Q same as P Q
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Semantics • Syntax: which arrangements of symbols are legal
– (Def “sentences”)
• Semantics: what the symbols mean in the world
– (Mapping between symbols and worlds)
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Sentences
Facts Facts
Sentences
Representation
World
Sem
antics
Sem
an
tics
Inference
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Propositional Logic: SEMANTICS
• “Interpretation” (or “possible world”)
– Assignment to each variable either T or F
– Assignment of T or F to each connective via defns
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P T
T
F
F
Q
P T
T
F
F
Q
P Q P Q
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Propositional Logic: SEMANTICS
• “Interpretation” (or “possible world”)
– Assignment to each variable either T or F
– Assignment of T or F to each connective via defns
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P T
T
F
F
Q
P T
T
F
F
Q
P Q P Q
T
F F
F
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Propositional Logic: SEMANTICS
• “Interpretation” (or “possible world”)
– Assignment to each variable either T or F
– Assignment of T or F to each connective via defns
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P T
T
F
F
Q
P T
T
F
F
Q
P Q P Q
T
F F
F
F
T T
T
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Satisfiability, Validity, & Entailment
• S is satisfiable if it is true in some world
• S is unsatisfiable if it is false all worlds
• S is valid if it is true in all worlds
• S1 entails S2 if wherever S1 is true S2 is also true
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Examples
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P Q
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Examples
R R
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P Q
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Examples
R R
S (W S)
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P Q
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Examples
R R
S (W S)
T T
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P Q
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Examples
R R
S (W S)
T T
X X
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P Q
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Notation
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=
Inference Entailment
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Notation
24
=
Inference Entailment
} Implication (syntactic symbol)
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Notation
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=
Inference Entailment
} Proves: S1 |-ie S2 if `ie’ algorithm says `S2’ from S1
Entails: S1 |= S2 if wherever S1 is true S2 is also true
Implication (syntactic symbol)
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Notation
• Sound
• Complete
• (all truth & nothing but the truth) =
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=
Inference Entailment
} Proves: S1 |-ie S2 if `ie’ algorithm says `S2’ from S1
Entails: S1 |= S2 if wherever S1 is true S2 is also true
Implication (syntactic symbol)
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Notation
• Sound
• Complete
• (all truth & nothing but the truth) =
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=
Inference Entailment
} Proves: S1 |-ie S2 if `ie’ algorithm says `S2’ from S1
Entails: S1 |= S2 if wherever S1 is true S2 is also true
= =
Implication (syntactic symbol)
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Reasoning Tasks • Model finding
KB = background knowledge
S = description of problem
Show (KB S) is satisfiable
A kind of constraint satisfaction
• Deduction
S = question
Prove that KB |= S
Two approaches:
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Reasoning Tasks • Model finding
KB = background knowledge
S = description of problem
Show (KB S) is satisfiable
A kind of constraint satisfaction
• Deduction
S = question
Prove that KB |= S
Two approaches:
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• Rules to derive new formulas from old (inference)
• Show (KB S) is unsatisfiable
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Special Syntactic Forms • General Form:
((q r) s)) (s t)
• Conjunction Normal Form (CNF)
( q r s ) ( s t)
Set notation: { ( q, r, s ), ( s, t) }
empty clause () = false
• Binary clauses: 1 or 2 literals per clause
( q r) ( s t)
• Horn clauses: 0 or 1 positive literal per clause
( q r s ) ( s t)
(qr) s (st) false 30
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Propositional Logic: Inference
A mechanical process for computing new sentences
1. Backward & Forward Chaining
2. Resolution (Proof by Contradiction)
3. Davis Putnam
4. WalkSat
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Inference 1: Forward Chaining
Forward Chaining Based on rule of modus ponens
If know P1, …, Pn & know (P1 ... Pn ) Q
Then can conclude Q
Backward Chaining: search
start from the query and go backwards
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Analysis
• Sound?
• Complete?
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Analysis
• Sound?
• Complete?
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Can you prove { } |= Q Q
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Analysis
• Sound?
• Complete?
• If KB has only Horn clauses & query is a single literal
– Forward Chaining is complete
– Runs linear in the size of the KB
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Can you prove { } |= Q Q
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Example
2
2
2
1
2
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Example
1
2
2
1
1
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Example
1
1
2
1
0
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Example
1
1
2
1
0
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Example
1
0
1
1
0
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Example
1
0
1
1
0
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Example
1
0
0
1
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Example
0
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0
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Example
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Propositional Logic: Inference A mechanical process for computing new sentences
1. Backward & Forward Chaining
2. Resolution (Proof by Contradiction)
3. GSAT
4. Davis Putnam
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Conversion to CNF
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Inference 2: Resolution [Robinson 1965]
{ (p ), ( p ) } |-R ( )
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Inference 2: Resolution [Robinson 1965]
{ (p ), ( p ) } |-R ( )
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Correctness
If S1 |-R S2 then S1 |= S2 Refutation Completeness:
If S is unsatisfiable then S |-R ()
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Resolution subsumes Modus Ponens
A B, A |= B
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Resolution subsumes Modus Ponens
A B, A |= B
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( A B)
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Resolution subsumes Modus Ponens
A B, A |= B
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( A B) (A)
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Resolution subsumes Modus Ponens
A B, A |= B
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( A B) (A)
(B)
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If Will goes, Jane will go ~W V J If doesn’t go, Jane will still go W V J Will Jane go? |= J?
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If Will goes, Jane will go ~W V J If doesn’t go, Jane will still go W V J Will Jane go? |= J?
J V J =J
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Don’t need to use other equivalences if we use resolution in refutation style ~J ~W ~W V J W V J
J
If Will goes, Jane will go ~W V J If doesn’t go, Jane will still go W V J Will Jane go? |= J?
J V J =J
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Resolution
56
If the unicorn is mythical, then it is immortal, but if
it is not mythical, it is a mammal. If the unicorn is either immortal or a mammal, then it is horned.
Prove: the unicorn is horned.
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Resolution
57
If the unicorn is mythical, then it is immortal, but if
it is not mythical, it is a mammal. If the unicorn is either immortal or a mammal, then it is horned.
Prove: the unicorn is horned.
M = mythical I = immortal A = mammal H = horned
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Resolution
58
If the unicorn is mythical, then it is immortal, but if
it is not mythical, it is a mammal. If the unicorn is either immortal or a mammal, then it is horned.
Prove: the unicorn is horned.
( A H)
(M A)
(I H)
( M I)
M = mythical I = immortal A = mammal H = horned
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Resolution
59
If the unicorn is mythical, then it is immortal, but if
it is not mythical, it is a mammal. If the unicorn is either immortal or a mammal, then it is horned.
Prove: the unicorn is horned.
( A H)
(M A)
( H) (I H)
( M I)
M = mythical I = immortal A = mammal H = horned
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Resolution
60
If the unicorn is mythical, then it is immortal, but if
it is not mythical, it is a mammal. If the unicorn is either immortal or a mammal, then it is horned.
Prove: the unicorn is horned.
( A H)
(M A)
( H) (I H)
( M I) (I) (A)
M = mythical I = immortal A = mammal H = horned
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Resolution
61
If the unicorn is mythical, then it is immortal, but if
it is not mythical, it is a mammal. If the unicorn is either immortal or a mammal, then it is horned.
Prove: the unicorn is horned.
( A H)
(M A)
( H) (I H)
( M)
( M I) (I) (A)
(M)
M = mythical I = immortal A = mammal H = horned
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Resolution
62
If the unicorn is mythical, then it is immortal, but if
it is not mythical, it is a mammal. If the unicorn is either immortal or a mammal, then it is horned.
Prove: the unicorn is horned.
( A H)
(M A)
( H) (I H)
( M)
( M I) (I) (A)
(M)
()
M = mythical I = immortal A = mammal H = horned
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Search in Resolution • Convert the database into clausal form Dc • Negate the goal first, and then convert it into clausal
form DG • Let D = Dc+ DG
• Loop – Select a pair of Clauses C1 and C2 from D
• Different control strategies can be used to select C1 and C2 to reduce number of resolutions tries
– Resolve C1 and C2 to get C12 – If C12 is empty clause, QED!! Return Success (We proved
the theorem; ) – D = D + C12
• Out of loop but no empty clause. Return “Failure” – Finiteness is guaranteed if we make sure that:
• we never resolve the same pair of clauses more than once;
• we use factoring, which removes multiple copies of literals from a clause (e.g. QVPVP => QVP)
Idea 1: Set of Support: At least one of C1 or C2 must be either the goal clause or a clause derived by doing resolutions on the goal clause (*COMPLETE*) Idea 2: Linear input form: Atleast one of C1 or C2 must be one of the clauses in the input KB (*INCOMPLETE*)
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Model Finding
• Find assignments to variables that makes a formula true
• a CSP
64
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Inference 3: Model Enumeration
for (m in truth assignments){
if (m makes true)
then return “Sat!”
}
return “Unsat!”
65
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Inference 4: DPLL (Enumeration of Partial Models) [Davis, Putnam, Loveland & Logemann 1962]
Version 1
dpll_1(pa){
if (pa makes F false) return false;
if (pa makes F true) return true;
choose P in F;
if (dpll_1(pa {P=0})) return true;
return dpll_1(pa {P=1});
}
Returns true if F is satisfiable, false otherwise
66
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DPLL Version 1
67
(a b c)
(a ¬b)
(a ¬c)
(¬a c)
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DPLL Version 1
68
a
(a b c)
(a ¬b)
(a ¬c)
(¬a c)
F
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DPLL Version 1
69
a
(F b c)
(F ¬b)
(F ¬c)
(T c)
F
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DPLL Version 1
70
a
(F F c)
(F T)
(F ¬c)
(T c)
F
b
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DPLL Version 1
71
a
(F F F)
(F T)
(F T)
(T F)
F
b
c
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DPLL Version 1
72
a
F
T
T
T
F
b
c
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DPLL Version 1
73
a
b
c
(a b c)
(a ¬b)
(a ¬c)
(¬a c)
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DPLL Version 1
74
a
b b
c c
(a b c)
(a ¬b)
(a ¬c)
(¬a c)
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DPLL as Search
• Search Space?
• Algorithm?
75
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Improving DPLL
76
1 1 2
1 1 2 3
2 3
1
If literal is true, then clause ( ...) is true
If clause is true, then ... has the
Therefore: Okay to delete clauses containing
s
tr
ame
value as ...
If lit
ue lit
eral is
erals!
L L L
C C C C
C C
L
1 2 3
2 3
1 1
Therefore: Okay to delete shorten containing false liter
false, then clause ( ...) has
the same value as ( ...)
If literal is false, then clause ( ) is fals
als!
Therefore: th
e
e empty clau
L L L
L L
L L
se means false!
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DPLL version 2
dpll_2(F, literal){
choose V in F;
if (dpll_2(F, V))return true;
return dpll_2(F, V);
}
77
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DPLL version 2
dpll_2(F, literal){
remove clauses containing literal
if (F contains no clauses)return true;
shorten clauses containing literal
if (F contains empty clause) return false;
choose V in F;
if (dpll_2(F, V))return true;
return dpll_2(F, V);
}
78
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DPLL Version 2
79
a
(F b c)
(F ¬b)
(F ¬c)
(T c)
F
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DPLL Version 2
80
a
(b c)
(¬b)
(¬c)
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DPLL Version 2
81
a
(F c)
(T)
(¬c)
b
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DPLL Version 2
82
a
(c)
(¬c)
b
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DPLL Version 2
83
a
(F)
(T)
b
c
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DPLL Version 2
84
a
()
b
c
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Structure in Clauses
85
• Unit Literals A literal that appears in a singleton clause {{b c}{c}{a b e}{d b}{e a c}}
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Structure in Clauses
86
• Unit Literals A literal that appears in a singleton clause {{b c}{c}{a b e}{d b}{e a c}}
Might as well set it true! And simplify {{b} {a b e}{d b}}
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Structure in Clauses
87
• Unit Literals A literal that appears in a singleton clause {{b c}{c}{a b e}{d b}{e a c}}
Might as well set it true! And simplify {{b} {a b e}{d b}}
{{d}}
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Structure in Clauses
• Pure Literals
– A symbol that always appears with same sign
– {{a b c}{c d e}{a b e}{d b}{e a c}}
88
• Unit Literals A literal that appears in a singleton clause {{b c}{c}{a b e}{d b}{e a c}}
Might as well set it true! And simplify {{b} {a b e}{d b}}
{{d}}
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Structure in Clauses
• Pure Literals
– A symbol that always appears with same sign
– {{a b c}{c d e}{a b e}{d b}{e a c}}
89
• Unit Literals A literal that appears in a singleton clause {{b c}{c}{a b e}{d b}{e a c}}
Might as well set it true! And simplify {{a b c} {a b e} {e a c}}
Might as well set it true! And simplify {{b} {a b e}{d b}}
{{d}}
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In Other Words
90
2 3
Therefore: Branch immediately on unit litera
Formula ( ) ... is only true when literal is true
If literal does not appear negated in formula , then setting
true preserves satisfiability o
ls!
L C C L
L F
L
Therefore: Branch immediately on pure liter
f
als!
F
May view this as adding constraint propagation techniques into play
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In Other Words
91
2 3
Therefore: Branch immediately on unit litera
Formula ( ) ... is only true when literal is true
If literal does not appear negated in formula , then setting
true preserves satisfiability o
ls!
L C C L
L F
L
Therefore: Branch immediately on pure liter
f
als!
F
May view this as adding constraint propagation techniques into play
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DPLL (previous version) Davis – Putnam – Loveland – Logemann
dpll(F, literal){
remove clauses containing literal
if (F contains no clauses) return true;
shorten clauses containing literal if (F contains empty clause) return false;
if (F contains a unit or pure L) return dpll(F, L);
choose V in F;
if (dpll(F, V))return true;
return dpll(F, V);
}
92
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DPLL (for real!) Davis – Putnam – Loveland – Logemann
dpll(F, literal){
remove clauses containing literal
if (F contains no clauses) return true;
shorten clauses containing literal if (F contains empty clause) return false;
if (F contains a unit or pure L) return dpll(F, L);
choose V in F;
if (dpll(F, V))return true;
return dpll(F, V);
}
93
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DPLL (for real)
94
a
b c
c
(a b c)
(a ¬b)
(a ¬c)
(¬a c)
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DPLL (for real!) Davis – Putnam – Loveland – Logemann
dpll(F, literal){
remove clauses containing literal
if (F contains no clauses) return true;
shorten clauses containing literal if (F contains empty clause) return false;
if (F contains a unit or pure L) return dpll(F, L);
choose V in F;
if (dpll(F, V))return true;
return dpll(F, V);
}
95
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Heuristic Search in DPLL
• Heuristics are used in DPLL to select a (non-unit, non-pure) proposition for branching
• Idea: identify a most constrained variable
96
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Heuristic Search in DPLL
• Heuristics are used in DPLL to select a (non-unit, non-pure) proposition for branching
• Idea: identify a most constrained variable
– Likely to create many unit clauses
97
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Heuristic Search in DPLL
• Heuristics are used in DPLL to select a (non-unit, non-pure) proposition for branching
• Idea: identify a most constrained variable
– Likely to create many unit clauses
• MOM’s heuristic:
– Most occurrences in clauses of minimum length
98
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Success of DPLL
• 1962 – DPLL invented
• 1992 – 300 propositions
• 1997 – 600 propositions (satz)
• Additional techniques:
– Learning conflict clauses at backtrack points
– Randomized restarts
– 2002 (zChaff) 1,000,000 propositions – encodings of hardware verification problems
99