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2013/04/13

real number system 7

We address how to handle \pm \infty in real number system. It is said to be a extended real number system.

In general, x\in \mathbb{R} means -\infty<x<+\infty. However, if x\in \mathbb{R} is not bounded above, we understand x=+\infty. It is convenient to make the rule of the fictitious number \infty.

["\infty"] for any x\in \mathbb{R}, (x\ne 0) , that is, -\infty<x<+\infty, (x\ne 0),
x+\infty=\infty, x-\infty=-\infty, \infty+\infty=\infty,
(\infty)\cdot (\infty)=\infty, (-\infty)\cdot (-\infty)=\infty, (-\infty)\cdot (\infty)=-\infty,
If x>0, then x\cdot (+\infty)=\infty, x\cdot (-\infty)=-\infty,
If x<0, then x\cdot (+\infty)=-\infty, x\cdot (-\infty)=\infty,
\frac{x}{+\infty}=0, \frac{x}{-\infty}=0,

Unfortunately we are not able to define the following forms.
\infty-\infty, \frac{\pm\infty}{\pm\infty}, 0\cdot\infty

Therefore, the extended real number system is ordered, but it is not a field. Do not take any notice of these definitions.

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