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Rushdi Shams, Dept of CSE, KUET, Bangladesh 1
Knowledge Representation
Propositional Logic
Artificial Intelligence
Version 2.0
There are 10 types of people in this world- who understand binary
and who do not understand binary
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Rushdi Shams, Dept of CSE, KUET, Bangladesh 3
Introduction
Need formal notation to represent knowledge,
allowing automated inference and problem solving.
One popular choice is use of logic.
Propositional logic is the simplest.
Symbols represent facts: P, Q, etc..
These are joined by logical connectives (and, or,
implication) e.g., P Λ Q; Q R
Given some statements in the logic we can deduce new
facts (e.g., from above deduce R)
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Rushdi Shams, Dept of CSE, KUET, Bangladesh 4
Syntactic Properties of
Propositional Logic
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
(bi-conditional)
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Rushdi Shams, Dept of CSE, KUET, Bangladesh 5
Semantic Properties of
Propositional Logic
S is true iff S is false
S1 S2 is true iff S1 is true and S2 is true
S1 S2 is true iff S1is true or S2 is true
S1 S2 is true iff S1 is false or S2 is true
i.e., is false iff S1 is true and S2 is false
S1 S2 is true iff S1 S2 is true and
S2 S1 is true
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Truth Table for Connectives
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Rushdi Shams, Dept of CSE, KUET, Bangladesh 7
Model of a Formula
If the value of the formula X holds 1 for the
assignment A, then the assignment A is called model
for formula X.
That means, all assignments for which the formula X
is true are models of it.
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Rushdi Shams, Dept of CSE, KUET, Bangladesh 8
Model of a Formula
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Rushdi Shams, Dept of CSE, KUET, Bangladesh 9
Model of a Formula:
Can you do it?
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Rushdi Shams, Dept of CSE, KUET, Bangladesh 10
Satisfiable Formulas
If there exist at least one model of a formula then the
formula is called satisfiable.
The value of the formula is true for at least one
assignment. It plays no rule how many models the
formula has.
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Rushdi Shams, Dept of CSE, KUET, Bangladesh 12
Valid Formulas
A formula is called valid (or tautology) if all
assignments are models of this formula.
The value of the formula is true for all assignments. If
a tautology is part of a more complex formula then
you could replace it by the value 1.
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Rushdi Shams, Dept of CSE, KUET, Bangladesh 14
Unsatisfiable Formulas
A formula is unsatisfiable if none of its
assignment is true in no models
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Rushdi Shams, Dept of CSE, KUET, Bangladesh 15
Logical equivalence
Two sentences are logically equivalent iff true in same models: α ≡ ß
iff α╞ β and β╞ α
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Rushdi Shams, Dept of CSE, KUET, Bangladesh 16
Deduction: Rule of Inference
1. Either cat fur was found at the scene of the crime, or dog fur was
found at the scene of the crime. (Premise)
C v D
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Rushdi Shams, Dept of CSE, KUET, Bangladesh 17
Deduction: Rule of Inference
2. If dog fur was found at the scene of the crime, then officer
Thompson had an allergy attack. (Premise)
D → A
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Deduction: Rule of Inference
3. If cat fur was found at the scene of the crime, then Macavity is
responsible for the crime. (Premise)
C → M
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Rushdi Shams, Dept of CSE, KUET, Bangladesh 19
Deduction: Rule of Inference
4. Officer Thompson did not have an allergy attack. (Premise)
¬ A
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Rushdi Shams, Dept of CSE, KUET, Bangladesh 20
Deduction: Rule of Inference
5. Dog fur was not found at the scene of the crime. (Follows from 2
D → A and 4. ¬ A). When is ¬ A true? When A is false- right?
Now, take a look at the implication truth table. Find what is the
value of D when A is false and D → A is true
¬ D
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Rushdi Shams, Dept of CSE, KUET, Bangladesh 21
Rules for Inference:
Modus Tollens
If given α → β
and we know ¬β
Then ¬α
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Rushdi Shams, Dept of CSE, KUET, Bangladesh 22
Deduction: Rule of Inference
6. Cat fur was found at the scene of the crime. (Follows from 1
C v D and 5 ¬ D). When is ¬ D true? When D is false- right?
Now, take a look at the OR truth table. Find what is the value of
C when D is false and C V D is true
C
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Rushdi Shams, Dept of CSE, KUET, Bangladesh 23
Rules for Inference:
Disjunctive Syllogism
If given α v β
and we know ¬α
then β
If given α v β
and we know ¬β
then α
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Deduction: Rule of Inference
7. Macavity is responsible for the crime. (Conclusion. Follows from
3 C → M and 6 C). When is C → M true given that C is true?
Take a look at the Implication truth table.
M
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Rules for Inference:
Modus Ponens
If given α → β
and we know α
Then β
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References
Artificial Intelligence: A Modern Approach (2nd
Edition)
by Russell and Norvig
Chapter 7
http://www.iep.utm.edu/p/prop-log.htm#H5
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