# example of quantifier

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or

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I include extracts of this Document below:

Example 1: let $p(x)$ be the property $0 where x is a real number. $p(1)$ is true and $p(0)$ is false.

Example 2: a property can have two or more variables. Let $p(x,y)$ be $x=y$. in this case $p(1,1)$ is true but $p(0,1)$ is false because $0$ is not equal to $1$.

Definition: let $p(x)$ be a property on the set $X$, i.e. $p(x)$ is a property and $x$ varies in the set $X$. a) The symbol $(\forall x\in X)(p(x))$ means for every $x$ in the set $X$ the proposition $p(x)$ is true. b) The symbol $(\exists x\in X)(p(x))$ means there is some $x$ in the set $X$ for which the proposition $p(x)$ is true. If $X=\emptyset$ , i.e. if the set $X$ is empty, $(\forall x\in X)(p(x))$ is defined to be true and $(\exists x\in X)(p(x))$ is defined to be false.

Example 1: $(\forall x\in\mathbb{R})(x=0\text{ or }x>0\text{ or }x<0)$ is a true proposition.

Example 2: $(\exists x\in\mathbb{R})(x^{2}+1=0)$ is false, because no real number satisfies $x^{2}+10=0$.

Example 3: $(\forall x\in\mathbb{R})(x is a property. $y$ varies in $\mathbb{R}$. As a result $(\forall x\in\mathbb{R})(\forall y\in\mathbb{R})(x is a proposition, i.e. it is a true or a false sentence. In fact $(\forall x\in\mathbb{R})(\forall y\in\mathbb{R})(x is false but $(\forall x\in\mathbb{R})(\forall y\in(x,\infty)(x is true; here $(x,\infty)$ is the interval containing real numbers greater than $x$.

some theorems:

for proofs of the following theorems see the address above

Theorem 1: if $(\forall x\in A)(p(x))$ and $(\forall x\in A)(p(x)\to q(x))$ then $(\forall x\in A)(q(x))$.

Theorem 22: if $(\exists y\in B)(\forall x\in A)(r(x,y))$ then $(\forall x\in A)(\exists y\in B)(r(x,y))$.

here $r(x,y)$ is a property on $A\times B$.

Title example of quantifier ExampleOfQuantifier 2013-05-23 19:14:17 2013-05-23 19:14:17 hkkass (6035) hkkass (6035) 19 hkkass (6035) Example msc 03B15 msc 03B10