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COMPOUND PROPOSITIONS
LECTURE3
LOGICAL EXPRESSIONS
 Any proposition must somehow be expressed
verbally, graphically, or by a string of characters.
 A proposition expressed by a string of characters is
called a logical expression or a formula.
 It can either be atomic or compound.
 Atomic expression is consisting of a single
propositional variable and it represents atomic
proposition.
 Compound expressions contain at least one
connective, and they represent compound
propositions.
 All expressions containing identifiers that
represent expressions are called schemas.
 If A = (P ^ Q) and if B = (P v Q), then the schema
(A => B) stands for ((P ^ Q) => (P v Q)).
PRECEDENCE RULES
1. ~
2. ^
3. v
4. =>
5. <=>
 ~P v Q is equal to (~P) v Q not ~(P v Q)
 P ^ Q v R is equal to (P ^ Q) v R
 P => Q v R is equal to P=> (Q v R)
 M <=> F => C is equal to M <=> (F => C)
 P => Q => R is equal to (P => Q) => R because all
binary logical connectives are left associative.
 Prefix –precedes it operand (start)
 Infix –inserted between the operands (middle)
 Postfix –follows its operands (end)
Examples of Infix, Prefix, and Postfix
Infix Expression Prefix Expression Postfix Expression
a + b , p ^ q + a b, ~p a b +
a + b * c , p ^ q v r + a * b c, ~q a b c * +
EVALUATION OF EXPRESSIONS
AND TRUTH TABLES
 “If you take a class in computer, and if you do not
understand logic, you will not pass.”
P: You take a class in computer.
Q: You understand logic.
R: You pass.
 We want to know exactly when this statement is
true or false. To do this we use the symbols P, Q,
and R. Using this, the statement in question
becomes (P ^ ~Q) => ~R.
TRUTH TABLE FOR (P ^ ~Q) => ~R:
P Q R ~Q P ^ ~Q ~R (P ^ ~Q)
=> ~R
T T T F F F T
T T F F F T T
T F T T T F F
T F F T T T T
F T T F F F T
F T F F F T T
F F T T F F T
F F F T F T T
SEATWORK
Given P and Q are true and R and S are false,
find the truth-values of the ff expressions:
1. P v (Q ^ R)
2. (P ^ (Q ^ R)) v ~((P v Q) ^ (R v S))
SOLUTION
1. P v (Q ^ R)
2. (P ^ (Q ^ R)) v ~((P v Q) ^ (R v S))
P Q R Q^R P v (Q ^ R)
T T F F T
T T F F T
T T F F T
T T F F T
P Q R S (Q ^
R)
(P ^ (Q ^
R))
(P v
Q)
(R v
S)
((P v Q) ^
(R v S))
~((P v Q) ^
(R v S))
v
T T F F F F T F F T T
T T F F F F T F F T T
T T F F F F T F F T T
T T F F F F T F F T T
TAUTOLOGIES AND
CONTRADICTIONS
LECTURE 4
 Tautology gives all answers with truth value of
TRUE.
 Contradiction gives all answers with truth
value of FALSE.
 Contingency - proposition that is neither a
tautology nor a contradiction
 Tautology and Logical equivalence
Definitions:
 A compound proposition that is always True is
called a tautology.
 Two propositions p and q are logically equivalent
if their truth tables are the same.
 Namely, p and q are logically equivalent if p ↔ q
is a tautology. If p and q are logically equivalent,
we write p ≡ q.
EXAMPLES OF LOGICAL EQUIVALENCE
 Example: Look at the following two compound
propositions: p → q and q ∨ ¬p.
 The last column of the two truth tables are
identical. Therefore (p → q) and (q ∨ ¬p) are
logically equivalent.
 So (p → q) ↔ (q ∨ ¬p) is a tautology.
 Thus: (p → q)≡ (q ∨ ¬p).
 Example:
 By using truth table, prove p ⊕ q ≡ ¬ (p ↔ q).
DE MORGAN LAW
 We have a number of rules for logical
equivalence.
 For example: De Morgan Law:
¬(p ∧ q) ≡ ¬p ∨ ¬q (1)
¬(p ∨ q) ≡ ¬p ∧ ¬q (2)
 The following is the truth table proof for (1). The
proof for (2) is similar
DISTRIBUTIVITY
p ∨ (q ∧ r) ≡ (p ∨ q) ∧ (p ∨ r) (1)
p ∧ (q ∨ r) ≡ (p ∧ q) ∨ (p ∧ r) (2)
The following is the truth table proof of (1). The
proof of (2) is similar.
CONTRAPOSITIVES
 The proposition ¬q → ¬p is called the
Contrapositive of the proposition p → q. They
are logically equivalent.
p → q ≡ ¬q → ¬p
LOGIC EQUIVALENCES
LOGICAL EQUIVALENCES INVOLVING
CONDITIONAL STATEMENTS
LOGICAL EQUIVALENCES INVOLVING
BICONDITIONAL STATEMENTS

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Lecture-3-and-4.pdf

  • 2. LOGICAL EXPRESSIONS  Any proposition must somehow be expressed verbally, graphically, or by a string of characters.  A proposition expressed by a string of characters is called a logical expression or a formula.  It can either be atomic or compound.  Atomic expression is consisting of a single propositional variable and it represents atomic proposition.  Compound expressions contain at least one connective, and they represent compound propositions.
  • 3.  All expressions containing identifiers that represent expressions are called schemas.  If A = (P ^ Q) and if B = (P v Q), then the schema (A => B) stands for ((P ^ Q) => (P v Q)).
  • 4. PRECEDENCE RULES 1. ~ 2. ^ 3. v 4. => 5. <=>  ~P v Q is equal to (~P) v Q not ~(P v Q)  P ^ Q v R is equal to (P ^ Q) v R  P => Q v R is equal to P=> (Q v R)  M <=> F => C is equal to M <=> (F => C)  P => Q => R is equal to (P => Q) => R because all binary logical connectives are left associative.
  • 5.  Prefix –precedes it operand (start)  Infix –inserted between the operands (middle)  Postfix –follows its operands (end) Examples of Infix, Prefix, and Postfix Infix Expression Prefix Expression Postfix Expression a + b , p ^ q + a b, ~p a b + a + b * c , p ^ q v r + a * b c, ~q a b c * +
  • 6. EVALUATION OF EXPRESSIONS AND TRUTH TABLES  “If you take a class in computer, and if you do not understand logic, you will not pass.” P: You take a class in computer. Q: You understand logic. R: You pass.  We want to know exactly when this statement is true or false. To do this we use the symbols P, Q, and R. Using this, the statement in question becomes (P ^ ~Q) => ~R.
  • 7. TRUTH TABLE FOR (P ^ ~Q) => ~R: P Q R ~Q P ^ ~Q ~R (P ^ ~Q) => ~R T T T F F F T T T F F F T T T F T T T F F T F F T T T T F T T F F F T F T F F F T T F F T T F F T F F F T F T T
  • 8. SEATWORK Given P and Q are true and R and S are false, find the truth-values of the ff expressions: 1. P v (Q ^ R) 2. (P ^ (Q ^ R)) v ~((P v Q) ^ (R v S))
  • 9. SOLUTION 1. P v (Q ^ R) 2. (P ^ (Q ^ R)) v ~((P v Q) ^ (R v S)) P Q R Q^R P v (Q ^ R) T T F F T T T F F T T T F F T T T F F T P Q R S (Q ^ R) (P ^ (Q ^ R)) (P v Q) (R v S) ((P v Q) ^ (R v S)) ~((P v Q) ^ (R v S)) v T T F F F F T F F T T T T F F F F T F F T T T T F F F F T F F T T T T F F F F T F F T T
  • 11.  Tautology gives all answers with truth value of TRUE.  Contradiction gives all answers with truth value of FALSE.  Contingency - proposition that is neither a tautology nor a contradiction
  • 12.  Tautology and Logical equivalence Definitions:  A compound proposition that is always True is called a tautology.  Two propositions p and q are logically equivalent if their truth tables are the same.  Namely, p and q are logically equivalent if p ↔ q is a tautology. If p and q are logically equivalent, we write p ≡ q.
  • 13. EXAMPLES OF LOGICAL EQUIVALENCE  Example: Look at the following two compound propositions: p → q and q ∨ ¬p.  The last column of the two truth tables are identical. Therefore (p → q) and (q ∨ ¬p) are logically equivalent.  So (p → q) ↔ (q ∨ ¬p) is a tautology.  Thus: (p → q)≡ (q ∨ ¬p).  Example:  By using truth table, prove p ⊕ q ≡ ¬ (p ↔ q).
  • 14. DE MORGAN LAW  We have a number of rules for logical equivalence.  For example: De Morgan Law: ¬(p ∧ q) ≡ ¬p ∨ ¬q (1) ¬(p ∨ q) ≡ ¬p ∧ ¬q (2)  The following is the truth table proof for (1). The proof for (2) is similar
  • 15. DISTRIBUTIVITY p ∨ (q ∧ r) ≡ (p ∨ q) ∧ (p ∨ r) (1) p ∧ (q ∨ r) ≡ (p ∧ q) ∨ (p ∧ r) (2) The following is the truth table proof of (1). The proof of (2) is similar.
  • 16. CONTRAPOSITIVES  The proposition ¬q → ¬p is called the Contrapositive of the proposition p → q. They are logically equivalent. p → q ≡ ¬q → ¬p