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概述

离散数学(Discrete Math)

                                           - 谓词逻辑

目录

离散数学(Discrete Math)

                                           - 谓词逻辑

Defs

Predicate Logic 谓词逻辑

Universal Quantifier 全称量词

Existential Quantifier 存在量词

Universal Quantifier VS Existential Quantifier 对比

The uniqueness quantifier  唯一量词 ∃!

Binding and Free Variables (静态变量和可变变量)

交换律和结合律


Defs

Predicate Logic 谓词逻辑

The area of logic that deals with predicates and quantitfiers is called the predicate calculus.

Ex:P(x,y,z) = “x=y+z”,故P(3,2,1) = True   P(1,2,1) = False

Universal Quantifier 全称量词

The universal quantification of P(x) is the statement “P(x) for all value of x in the domain.”

∀ is called the universal quantifier.

∀x P(x) is read “for all x P(x) ” or ”for every x P(x)” or ”任取x对于P(x)……”

Key concept :

The Domain 语集/论域 .The domain of discourse , and universal of discourse.

Existential Quantifier 存在量词

The existential quantification of P(x) is the statement “ There exists an element x in the domain such that P(x).”

∃ is called the existential quantifier.

∃x P(x) is read “ There exists an element x in the domain such that P(x). ” or ” 存在x对于P(x)…… ”

The statement is false if and only if ∀x ┐P(x) , i.e., (后接内容解释上述内容,e.g.是for example的意思)

┐(∃x P(x)) ≡ ∀x ┐P(x)

Universal Quantifier VS Existential Quantifier 对比

Statement

When true

When false

∀x P(x)

P(x) is true for every x

There is an x for which P(x) is false

∃x P(x)

There is an x for which P(x) is false

P(x) is false for every x

 

The uniqueness quantifier  唯一量词 ∃!

∃! x P(x) is the statement “There exists exactly one x such that P(x) is true.”

优先级:∀和∃要高于之前所有的优先级。

例:∀x P(x)∨Q(x) != ∀x ( P(x)∨Q(x) )

Binding and Free Variables (静态变量和可变变量)

Statement

Binding and Free Variables

P(x)

X is free variable

P(x,y)

X,y are free variables

∀x P(x)

X is bound

∀x P(x,y)

y is free variables , and x is bound

∀x ∃yP(x,y)

x,y are bound 

交换律和结合律

statement

Result

∀x ∀y P(x,y)

∀y ∀x P(x,y) ≡ ∀xy P(x,y)

∃x ∃y P(x,y)

∃y ∃x P(x,y) ≡ ∃xy P(x,y)

∀x ∃yP(x,y)

× 对任意x都存在一个y……

∃y ∀x P(x,y)

× 存在一个y,使任意x对其…

∃x ∀y P(x,y)

× 存在一个x对任意y……

∀y ∃x P(x,y)

× 对任意的y,都有一个x……

 

 

 

 

 

 

最后

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