(述語論理、全称限量子、存在限量子)

http://www.sw.it.aoyama.ac.jp/2017/Math1/lecture6.html

© 2005-17 Martin J. Dürst Aoyama Gakuin University

- Schedule for the next few weeks
- Summary/homework for last lecture
- Predicates
- Quantifiers
- Laws for Quantifiers
- This week's homework

- October 27 (today): Predicate Logic, Quantifiers
- November 3:
**No lectures**(Aoyama Festival) - November 10: Application of Predicate Logic
- (regular weekly lectures after this date)

- All Boolean formulæ can be expressed using only NAND (⊼) or only NOR (⊽).
- Logic circuits can be built from gates to implement Boolean functions.
- The main gates are AND, OR, NOT, NAND, NOR, XOR (⊕).
- There are many different ways to axiomatize Boolean logic (learn at least one set of axioms).
- Logical operations important for symbolic logic are implication (→) and equivalence (↔).

- A Boolean formula that is always true is called a
`tautology`. - A Boolean formula that is always false is called a
`contradiction`. - All laws are tautologies, but there are also tautologies that we don't
call laws.

(Example: T→`A`= ¬¬`A`)

- Binary (Boolean) logic (using only true and false)
- Multi-valued logic (using e.g. true, false, and
`unknown`) - Fuzzy logic (including calculation of ambiguity)

- Propositional logic (using only prepositions)
- Predicate logic (first order predicate logic,...)
- Temporal logic (integrating temporal relationships)

With propositions, related statements have to be made separately

Examples:

Today it is sunny. Tomorrow it is sunny. The day after tomorrow, it is
sunny.

2 is even. 5 is even.

We can express "If today is sunny, then tomorrow will also be sunny." or "If 2 is even, then 3 is not even".

But we cannot express "If it's sunny on a given day, it's also sunny on the
next day." or "If x is even, then x+2 is also even.".

The problem with propositions can be solved by introducing
*predicates*.

In the same way as propositions, predicates are objectively true or false.

A predicate is a function (with 0 or more arguments) that returns true or false.

If the value of an argument is undefined, the result (value) of the predicate is unknown.

A predicate with 0 arguments is a proposition.

sunny(today), sunny(tomorrow), sunny(yesterday), even(2), even(5), ...

Generalization: sunny(`x`), even(`y`), ...

Using predicates, we can express new things:

- sunny(
`x`) → sunny(day after`x`) - even(
`y`) → even(`y`+2)

Similar to propositions, predicates can be true or false.

But predicates can also be unknown/undefined, for example if they contain variables.

Also, even if a predicate is undefined (e.g. even(`x`)),

a formula containing this predicate can be defined

(true, e.g. even(`y`) → even(`y`+2), or false, e.g.
odd(`z`) → even(`z`+24))

- The arguments of predicates can be constants, functions, formulæ,...
Examples:

- even(2), say(Romeo, 'I love you'), father(Ieyasu, Hidetada)
- even(sin(0)), even(2+3×7)

- However, it is not possible to use predicates within predicates

Counterexample: say(`z`, father(`y`,`x`))

(`z`says "`y`is the father of`x`") *Higher-order logic*allows predicates within predicates

Example: ∀`n`∈ℕ: even (`n`) →
even(`n`+2)

Readings:

- For all
`n`, elements of ℕ, if`n`is even, then`n`+2 is even. - For all natural numbers
`n`, if`n`is even, then`n`+2 is even.

General form: ∀`x`: P (`x`)

∀ is the **A** of "for **A**ll", inverted.

Readings in Japanese:

- 全ての自然数
`n`において、`n`が偶数ならば`n`+2 も偶数である - 任意の x において、P(
`x`)

∀`x`∈{}: P(`x`) = T

Reason: We have to check P(`x`) for all elements `x` in
the set.

If we find even one `x` where P(`x`) is false, the overall
statement is false.

But we cannot find any `x` where P(`x`) is false.

Application example:

All students in this room from Hungary are over 50 (years old).

See: Vacuous truth, https://en.wikipedia.org/wiki/Vacuous_truth

Example: ∃`n`∈ℕ: odd (`n`)

Readings:

- There exists a
`n`, element of ℕ, for which`n`is odd. - There is a natural number
`n`so that`n`is odd. - There exists a natural number
`n`which is odd. - There exists an odd natural number.

General form: ∃`y`: P (`y`)

∃ is the mirrored form of the **E** in "there
**E**xists".

Readings in Japanese:

- P(
`y`) が成立する`y`が存在する - ある
`y`について、P(`y`)

∀`n`∈ℕ: `n` + `n` + `n` =
3`n`

∃`n`∈ℕ: `n`^{2} =
`n`^{3}

∃`n`∈ℕ: `n`^{2} < 50`n` <
`n`^{3}

∀`m`, `n`∈ℕ: 7`m` + 2`n` =
2`n` + 7`m`

Peano Axioms (Guiseppe Peano, 1858-1932)

- 1∈ℕ
- ∀
`a`∈ℕ:`s`(`a`)∈ℕ - ¬∃
`x∈ℕ`:`s`(`x`) = 1 - ∀
`a`,`b`∈ℕ:`a`≠`b`→`s`(`a`) ≠`s`(`b`) - P(1) ∧ (∀
`a`∈ℕ: (P(a)→P(`s`(`a`)))) ⇒ ∀`a`∈ℕ: P(a)

- ¬∀
`x`: P(`x`) = ∃`x`: ¬P(`x`) - ¬∃
`x`: P(`x`) = ∀`x`: ¬P(`x`) - (
`X`≠{}∧∀`x`∈`X`: P(`x`)) → (∃`x`: P(`x`)) - (∀
`x`: P(`x`)) ∧ Q(`y`) = ∀`x`: P(`x`)∧Q(`y`) - (∃
`x`: P(`x`)) ∧ Q(`y`) = ∃`x`: P(`x`)∧Q(`y`) - (∀
`x`: P(`x`)) ∨ Q(`y`) = ∀`x`: P(`x`)∨Q(`y`) - (∃
`x`: P(`x`)) ∨ Q(`y`) = ∃`x`: P(`x`)∨Q(`y`) - (∀
`x`: P(`x`)) ∧ (∀`x`: R(`x`)) = ∀`x`: P(`x`)∧R(`x`) - (∀
`x`: P(`x`)) ∨ (∀`x`: R(`x`)) ⇒ ∀`x`: P(`x`)∨R(`x`) - (∃
`x`: P(`x`)) ∨ (∃`x`: R(`x`)) = ∃`x`: P(`x`)∨R(`x`) - (∃
`x`: P(`x`)) ∧ (∃`x`: R(`x`)) ⇐ ∃`x`: P(`x`)∧R(`x`) - (∃
`y`: ∀`x`: P(`x`,`y`)) ⇒ (∀`x`: ∃`y`: P(`x`,`y`)) - P(x) is a tautology ⇔∀
`x`: P(`x`) is a tautology

Deadline: **November 9**, 2017 (Thursday), 19:00.

Format: **A4 single page** (using both sides is okay; NO cover
page; an additional page is okay if really necessary, but staple the pages
together at the top left corner), easily readable **handwriting**
(NO printouts), name (kanji and kana) and student number at the top right

Where to submit: Box in front of room O-529 (building O, 5th floor)

Problem 1: Show that the Wolfram axiom of Boolean logic is a tautology (you can use either a truth table or formula manipulation).

Problem 2: For ternary (three-valued) logic, create truth tables for
conjunction, disjunction, and negation. The three values are T, F, and
**?**, where **?** stands for "unknown" (in more
words: "maybe true, maybe false, we don't know").

Hint: What's the result of "**?**∨T"? **?** can
be T or F, but in both cases the result will be T, so
**?**∨T=T.

Problem 3: For each of the laws 1, 5, 8, 11, and 12 of "Laws for Quantifiers", imagine a concrete example and explain it. For laws 11 and 12, give examples for both why the implication works one way and why the implication does not work the other way.

- predicate logic
- 述語論理
- quantifier
- 限量子
- evaluate
- 評価する
- evaluation
- 評価
- array
- 配列
- tautology
- 恒真 (式)、トートロジー
- contradiction
- 恒偽 (式)
- symbolic logic
- 記号論理
- multi-valued logic
- 多値論理
- fuzzy logic
- ファジィ論理
- ambiguity
- 曖昧さ
- first-order predicate logic
- 一階述語論理
- temporal logic
- 時相論理
- binary logic
- 二値論理
- generalization
- 一般化
- undefined
- 未定
- higher-order logic
- 高階述語論理
- universal quantifier
- 全称限量子 (全称記号)
- existential quantifier
- 存在限量子 (存在記号)