Propositions and Boolean Formulæ

(命題と論理式)

Discrete Mathematics I

3rd lecture, October 3, 2014

http://www.sw.it.aoyama.ac.jp/2014/Math1/lecture3.html

Martin J. Dürst

AGU

© 2005-14 Martin J. Dürst Aoyama Gakuin University

Today's Schedule

Summary of Last Lecture

Last Week's Homework: Jokes

Question: Why do computer scientist always think Christmas and Halloween are the same?

[都合により削除]

Question: At what age do Information Technologists celebrate "Kanreki" (還暦)

[都合により削除]

 

Last Week's Homework: Arithmetic and Base Conversion

About Moodle

Those who have not yet registered for "Discrete Mathematics I" at http://moo.sw.it.aoyama.ac.jp must come to the front immediately after this lecture

[まだ http://moo.sw.it.aoyama.ac.jp で授業登録できてない人は授業直後申し出ること!]

Proposition

A proposition is a sentence that is objectively either correct or not.

Propositions may also be called statements.

Even if a sentence is not correct, it is a proposition.

Even if we do not know the answer (but there is a single answer), it is a proposition.

Examples:

 

Counterexamples of Propositions

The following are NOT propositions:

Counterexamples (NOT propositions):

 

Truth Values of a Proposition

Each proposition is either correct or not.

"Correct" is called true (真), and is written T, ⊤, or 1.

"Not correct" is called false (偽), and is written F, ⊥, or 0.

True and false together are called truth values.

Other representations for truth values: ○/×,...

 

Truth Values in Program Languages

The handling of truth values (also called booleans) differs for different programming languages:

 

The Importance of Logical Operations

In Information Technology:

In Mathematics:

Modeling

Logical Operations

With logical operations, we can create compound propositions from simpler propositions.

The most frequently used logical operations are as follows:

Conjunction (and)

Based on two propositions A and B, we can construct the following proposition:

A and B

We write AB (also: A·B, A B, AND(A, B))

Because ∧ has two operands, it is a binary operation.

AB is defined as follows:
If both A and B are true, then AB is true.
Else, AB is false.

Examples:
4 is smaller than 5 and is even. T
7 is smaller than 5 and is even. F
8 is smaller than 5 and is even. F
3 is smaller than 5 and is even. F

 

Truth Table for Logical And

A B AB
F F F
F T F
T F F
T T T

Disjunction (or)

Based on two propositions A and B, we can construct the following proposition:

A or B

A or B is written AB (also: A+B, OR(A, B))

AB is defined as follows:
If both A and B are false, then AB is false.
Else, AB is true.

This means that logical or is inclusive.

Examples:
4 is smaller than 5 or is even. T
8 is smaller than 5 or is even. T
3 is smaller than 5 or is even. T
7 is smaller than 5 or is even. F

Truth Table for Logical Or

A B AB
F F F
F T T
T F T
T T T

 

Negation

Based on one proposition A, we can construct the following proposition:

not A

not A is written ¬A (also A', A, ~A, NOT(A)).

¬A is true if A is false, and is false if A is true.

Negation has only one operand and is therefore an unary operation.

Examples:
4 is not even. F
4 is not odd. T

Truth Table for Logical Not

A ¬A
F T
T F

 

Structure of Logical Formulæ

We can creat a logical formula from propositions, propositional variables, and logical operators.

Example: (A ∨ (¬B)) ∧ C

(operator) precedence and omission of parentheses:

For logical operators, ¬ has higher precedence than ∧, which has higher precedence than ∨.

Example: ABC = A ∨ (BC) ≠ (AB) ∧ C

 

Well-Formed Formula

(WFF)

Goal: Make the structure (grammar) of logical formulæ clear.

All of the following are well-formed formulæ

Formulæ that do not fit the above definition are not well-formed formulæ.
(Attention: In a later lecture, we will introduce more logical operators.)

 

Logical Formula Evaluation with a Truth Table

Truth tables are often used for:

Example formula: (A ∨ (¬B)) ∧ B

Simplification (the parentheses around ¬B are unnecessary): (A ∨ ¬B) ∧ B

Evaluation Using a Truth Table

A B ¬B A ∨ ¬B (A ∨ ¬B) ∧ B
F F T T F
F T F F F
T F T T F
T T F T T

 

How to Write a Truth Table

How to Use a Truth Table for a Proof

A B AB AAB
F F F F
F T F F
T F F T
T T T T

Boolean Functions

This Week's Homework

  1. Solve the quiz Propositions: True or False until October 9, 22:00
  2. Solve the quiz Truth Table 1 until October 9, 22:00
  3. Solve the quiz Truth Table 2 until October 9, 22:00

Deadline: October 9, 22:00

Repeat each quiz until you get 100% correct answers.

今回の宿題

  1. Moodle で Propositions: True or False のクイズを解く (全正解でない場合、繰り返す、締切: 10月 9日 (木) 22:00)
  2. Moodle で Truth Table 1 のクイズを解く (全正解でない場合、繰り返す、締切: 10月 9日 (木) 22:00)
  3. Moodle で Truth Table 2 のクイズを解く (全正解でない場合、繰り返す、締切: 10月 9日 (木) 22:00)

Glossary

round number
切りのいい数
proposition
命題
objective (adj.)
客観的
subjective (adj.)
主観的
opinion
意見
variable
変数
pronoun
代名詞
odd number
奇数
even number
偶数
truth value (of a proposition)
(命題の) 真偽
true
真 (しん)
false
偽 (ぎ)
type (in programming languages)
(プログラム言語の) 型
logical operation
論理演算
electronic circuits
電子回路
reasoning
推理、推論
modeling
モデル化 (する)
(logical) and
かつ
binary operation
二項演算
unary operation
単項演算
conjunction
論理積
truth table
真理表
logical operation
論理演算
compound proposition
複合命題
(logical) or
又は
disjunction
論理和
(logical) not
ではない
negation
論理否定
logical formula
論理式 (複数: formulæ or formulas)
well-formed (logical) formula
整論理式
grammar
文法
propositional variable
命題変数
(operator) precedence
(演算子の) 優先順位
evaluation
(式の) 評価
parentheses
括弧
subexpression
部分式
Boolean function
論理関数、ブール関数
Boolean argument
論理引数 (ひきすう; ブール引数とも言う)
domain (of a function)
(関数の) 定義域
range (of a function)
(関数の) 値域