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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