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Let’s Talk About Logic

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- Logic is a system based on propositions.
- A proposition is a statement that is either true or false (not both).
- We say that the truth value of a proposition is either true (T) or false (F).
- Corresponds to 1 and 0 in digital circuits

Applied Discrete Mathematics

Week 1: Logic and Sets

- Negation (NOT)
- Conjunction (AND)
- Disjunction (OR)
- Exclusive or (XOR)
- Implication (if – then)
- Biconditional (if and only if)
- Truth tables can be used to show how these operators can combine propositions to compound propositions.

Applied Discrete Mathematics

Week 1: Logic and Sets

- A tautology is a statement that is always true.
- Examples:
- R(R)
- (PQ)(P)(Q)
- If ST is a tautology, we write ST.
- If ST is a tautology, we write ST.

Applied Discrete Mathematics

Week 1: Logic and Sets

- A contradiction is a statement that is alwaysfalse.
- Examples:
- R(R)
- ((PQ)(P)(Q))
- The negation of any tautology is a contradiction, and the negation of any contradiction is a tautology.

Applied Discrete Mathematics

Week 1: Logic and Sets

- Propositional function (open sentence):
- statement involving one or more variables,
- e.g.: x-3 > 5.
- Let us call this propositional function P(x), where P is the predicate and x is the variable.

What is the truth value of P(2) ?

false

What is the truth value of P(8) ?

false

What is the truth value of P(9) ?

true

Applied Discrete Mathematics

Week 1: Logic and Sets

- Let us consider the propositional function Q(x, y, z) defined as:
- x + y = z.
- Here, Q is the predicate and x, y, and z are the variables.

What is the truth value of Q(2, 3, 5) ?

true

What is the truth value of Q(0, 1, 2) ?

false

What is the truth value of Q(9, -9, 0) ?

true

Applied Discrete Mathematics

Week 1: Logic and Sets

- Let P(x) be a propositional function.
- Universally quantified sentence:
- For all x in the universe of discourse P(x) is true.
- Using the universal quantifier :
- x P(x) “for all x P(x)” or “for every x P(x)”
- (Note: x P(x) is either true or false, so it is a proposition, not a propositional function.)

Applied Discrete Mathematics

Week 1: Logic and Sets

- Example:
- S(x): x is a UMB student.
- G(x): x is a genius.
- What does x (S(x) G(x)) mean ?
- “If x is a UMB student, then x is a genius.”
- or
- “All UMB students are geniuses.”

Applied Discrete Mathematics

Week 1: Logic and Sets

- Existentially quantified sentence:
- There exists an x in the universe of discourse for which P(x) is true.
- Using the existential quantifier :
- x P(x) “There is an x such that P(x).”
- “There is at least one x such that P(x).”
- (Note: x P(x) is either true or false, so it is a proposition, but no propositional function.)

Applied Discrete Mathematics

Week 1: Logic and Sets

- Example:
- P(x): x is a UMB professor.
- G(x): x is a genius.
- What does x (P(x) G(x)) mean ?
- “There is an x such that x is a UMB professor and x is a genius.”
- or
- “At least one UMB professor is a genius.”

Applied Discrete Mathematics

Week 1: Logic and Sets

- Another example:
- Let the universe of discourse be the real numbers.
- What does xy (x + y = 320) mean ?
- “For every x there exists a y so that x + y = 320.”

Is it true?

yes

Is it true for the natural numbers?

no

Applied Discrete Mathematics

Week 1: Logic and Sets

- A counterexample to x P(x) is an object c so that P(c) is false.
- Statements such as x (P(x) Q(x)) can be disproved by simply providing a counterexample.

Statement: “All birds can fly.”

Disproved by counterexample: Penguin.

Applied Discrete Mathematics

Week 1: Logic and Sets

- (x P(x)) is logically equivalent to x (P(x)).
- (x P(x)) is logically equivalent to x (P(x)).
- See Table 3 in Section 1.3.
- I recommend exercises 5 and 9 in Section 1.3.

Applied Discrete Mathematics

Week 1: Logic and Sets

- Set Theory

Actually, you will see that logic and set theory are very closely related.

Applied Discrete Mathematics

Week 1: Logic and Sets

- Set: Collection of objects (“elements”)
- aA “a is an element of A” “a is a member of A”
- aA “a is not an element of A”
- A = {a1, a2, …, an} “A contains…”
- Order of elements is meaningless
- It does not matter how often the same element is listed.

Applied Discrete Mathematics

Week 1: Logic and Sets

- Sets A and B are equal if and only if they contain exactly the same elements.
- Examples:

- A = {9, 2, 7, -3}, B = {7, 9, -3, 2} :

A = B

- A = {dog, cat, horse}, B = {cat, horse, squirrel, dog} :

A B

- A = {dog, cat, horse}, B = {cat, horse, dog, dog} :

A = B

Applied Discrete Mathematics

Week 1: Logic and Sets

- “Standard” Sets:
- Natural numbers N = {0, 1, 2, 3, …}
- Integers Z = {…, -2, -1, 0, 1, 2, …}
- Positive Integers Z+ = {1, 2, 3, 4, …}
- Real Numbers R = {47.3, -12, , …}
- Rational Numbers Q = {1.5, 2.6, -3.8, 15, …}(correct definition will follow)

Applied Discrete Mathematics

Week 1: Logic and Sets

- A = “empty set/null set”
- A = {z} Note: zA, but z {z}
- A = {{b, c}, {c, x, d}}
- A = {{x, y}} Note: {x, y} A, but {x, y} {{x, y}}
- A = {x | P(x)}“set of all x such that P(x)”
- A = {x | xN x > 7} = {8, 9, 10, …}“set builder notation”

Applied Discrete Mathematics

Week 1: Logic and Sets

- We are now able to define the set of rational numbers Q:
- Q = {a/b | aZ bZ+}
- or
- Q = {a/b | aZ bZ b0}
- And how about the set of real numbers R?
- R = {r | r is a real number}That is the best we can do.

Applied Discrete Mathematics

Week 1: Logic and Sets

- A B “A is a subset of B”
- A B if and only if every element of A is also an element of B.
- We can completely formalize this:
- A B x (xA xB)
- Examples:

A = {3, 9}, B = {5, 9, 1, 3}, A B ?

true

A = {3, 3, 3, 9}, B = {5, 9, 1, 3}, A B ?

true

A = {1, 2, 3}, B = {2, 3, 4}, A B ?

false

Applied Discrete Mathematics

Week 1: Logic and Sets

U

B

C

A

- Useful rules:
- A = B (A B) (B A)
- (A B) (B C) A C (see Venn Diagram)

Applied Discrete Mathematics

Week 1: Logic and Sets

- Useful rules:
- A for any set A
- A A for any set A
- Proper subsets:
- A B “A is a proper subset of B”
- A B x (xA xB) x (xB xA)
- or
- A B x (xA xB) x (xB xA)

Applied Discrete Mathematics

Week 1: Logic and Sets

- If a set S contains n distinct elements, nN,we call S a finite set with cardinality n.
- Examples:
- A = {Mercedes, BMW, Porsche}, |A| = 3

B = {1, {2, 3}, {4, 5}, 6}

|B| = 4

C =

|C| = 0

D = { xN | x 7000 }

|D| = 7001

E = { xN | x 7000 }

E is infinite!

Applied Discrete Mathematics

Week 1: Logic and Sets

- 2Aor P(A) “power set of A”
- 2A = {B | B A} (contains all subsets of A)
- Examples:
- A = {x, y, z}
- 2A = {, {x}, {y}, {z}, {x, y}, {x, z}, {y, z}, {x, y, z}}
- A =
- 2A = {}
- Note: |A| = 0, |2A| = 1

Applied Discrete Mathematics

Week 1: Logic and Sets

- Cardinality of power sets:
- | 2A | = 2|A|
- Imagine each element in A has an “on/off” switch
- Each possible switch configuration in A corresponds to one element in 2A

- For 3 elements in A, there are 222 = 8 elements in 2A

Applied Discrete Mathematics

Week 1: Logic and Sets

- The ordered n-tuple (a1, a2, a3, …, an) is an ordered collection of objects.
- Two ordered n-tuples (a1, a2, a3, …, an) and (b1, b2, b3, …, bn) are equal if and only if they contain exactly the same elements in the same order, i.e. ai = bi for 1 i n.
- The Cartesian product of two sets is defined as:
- AB = {(a, b) | aA bB}
- Example: A = {x, y}, B = {a, b, c}AB = {(x, a), (x, b), (x, c), (y, a), (y, b), (y, c)}

Applied Discrete Mathematics

Week 1: Logic and Sets

- Note that:
- A =
- A =
- For non-empty sets A and B: AB AB BA
- |AB| = |A||B|
- The Cartesian product of two or more sets is defined as:
- A1A2…An = {(a1, a2, …, an) | aiA for 1 i n}

Applied Discrete Mathematics

Week 1: Logic and Sets

- Union: AB = {x | xA xB}
- Example: A = {a, b}, B = {b, c, d}
- AB = {a, b, c, d}
- Intersection: AB = {x | xA xB}
- Example: A = {a, b}, B = {b, c, d}
- AB = {b}

Applied Discrete Mathematics

Week 1: Logic and Sets