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Definition/Summary
Let [itex]\mathbb{R}[/itex] be the set of all real numbers. We can extend [itex]\mathbb{R}[/itex] by adjoining two elements [itex]+\infty[/itex] and [itex]-\infty[/itex]. This forms the extended real number system. In notation:
[tex]\overline{\mathbb{R}}:=\mathbb{R}\cup \{+\infty,-\infty\}[/tex]
The extended real numbers are being introduced to give an interpretation to limits such as [itex]\lim_{x\rightarrow +\infty}{f(x)}[/itex]. Without introducing extended reals, the notation [itex]x\rightarrow +\infty[/itex] would just be a notation, nothing more. But after introducing the extended reals, we can work with [itex]+\infty[/itex] like other numbers.
Equations
Extended explanation
As stated in the introduction, we define the extended real numbers as
[tex]\overline{\mathbb{R}}:=\mathbb{R}\cup \{+\infty,-\infty\}[/tex]
We can define arithmetic on the extended reals. If [itex]a\neq -\infty[/itex] is an extended real, then we set
[tex]a+(+\infty)=+\infty[/tex]
The operation [itex](-\infty)+(+\infty)[/itex] is left undefined. Likewise, if [itex]a\neq +\infty[/itex] is an extended real, we set
[tex]a+(-\infty)=-\infty[/tex]
Multiplication is defined very similarly, for example
[tex]2\cdot (+\infty)=+\infty[/tex]
The only limitation is that [itex]0\cdot (+\infty)[/itex] and [itex]0\cdot (-\infty)[/itex] are left undefined. Note that this implies that
[tex]\frac{1}{+\infty}=\frac{1}{-\infty}=0[/tex]
But division by 0 is still undefined.
We can extend the order of [itex]\mathbb{R}[/itex] by setting
[tex]-\infty<a<+\infty[/tex]
for [itex]a\in \mathbb{R}[/itex].
All of this was fairly obvious, but the fun starts when we want to define a metric on the extended reals. For this we define the extended arctangent function as
[tex]\overline{atan}(x):\overline{\mathbb{R}} \rightarrow [-\frac{\pi}{2},\frac{\pi}{2}]:x\rightarrow \left\{\begin{array}{ccc}
-\frac{\pi}{2} & \text{if} & x=-\infty\\
\frac{\pi}{2} & \text{if} & x=+\infty\\
atan(x) & \text{if} & \text{otherwise}\\
\end{array}\right.[/tex]
Now, we define the following metric;
[tex]d:\overline{\mathbb{R}}\times\overline{\mathbb{R}}\rightarrow \mathbb{R}:(x,y)\rightarrow|\overline{atan}(x)-\overline{atan}(y)|[/tex]
With this definition, the extended reals become a compact, connected metric space. This definition also allows us to define limits and continuity in the ordinary sense, and these limits correspond to the limits without extended reals. For example [itex]\lim_{x\rightarrow +\infty}{f(x)}=a[/itex] in the extended reals if and only if
[tex]\forall \varepsilon >0:~ \exists P:~\forall x\geq P:~|f(x)-a|<\varepsilon[/tex]
As a topological space, the extended reals are homeomorph to [0,1].
* This entry is from our old Library feature. If you know who wrote it, please let us know so we can attribute a writer. Thanks!
Let [itex]\mathbb{R}[/itex] be the set of all real numbers. We can extend [itex]\mathbb{R}[/itex] by adjoining two elements [itex]+\infty[/itex] and [itex]-\infty[/itex]. This forms the extended real number system. In notation:
[tex]\overline{\mathbb{R}}:=\mathbb{R}\cup \{+\infty,-\infty\}[/tex]
The extended real numbers are being introduced to give an interpretation to limits such as [itex]\lim_{x\rightarrow +\infty}{f(x)}[/itex]. Without introducing extended reals, the notation [itex]x\rightarrow +\infty[/itex] would just be a notation, nothing more. But after introducing the extended reals, we can work with [itex]+\infty[/itex] like other numbers.
Equations
Extended explanation
As stated in the introduction, we define the extended real numbers as
[tex]\overline{\mathbb{R}}:=\mathbb{R}\cup \{+\infty,-\infty\}[/tex]
We can define arithmetic on the extended reals. If [itex]a\neq -\infty[/itex] is an extended real, then we set
[tex]a+(+\infty)=+\infty[/tex]
The operation [itex](-\infty)+(+\infty)[/itex] is left undefined. Likewise, if [itex]a\neq +\infty[/itex] is an extended real, we set
[tex]a+(-\infty)=-\infty[/tex]
Multiplication is defined very similarly, for example
[tex]2\cdot (+\infty)=+\infty[/tex]
The only limitation is that [itex]0\cdot (+\infty)[/itex] and [itex]0\cdot (-\infty)[/itex] are left undefined. Note that this implies that
[tex]\frac{1}{+\infty}=\frac{1}{-\infty}=0[/tex]
But division by 0 is still undefined.
We can extend the order of [itex]\mathbb{R}[/itex] by setting
[tex]-\infty<a<+\infty[/tex]
for [itex]a\in \mathbb{R}[/itex].
All of this was fairly obvious, but the fun starts when we want to define a metric on the extended reals. For this we define the extended arctangent function as
[tex]\overline{atan}(x):\overline{\mathbb{R}} \rightarrow [-\frac{\pi}{2},\frac{\pi}{2}]:x\rightarrow \left\{\begin{array}{ccc}
-\frac{\pi}{2} & \text{if} & x=-\infty\\
\frac{\pi}{2} & \text{if} & x=+\infty\\
atan(x) & \text{if} & \text{otherwise}\\
\end{array}\right.[/tex]
Now, we define the following metric;
[tex]d:\overline{\mathbb{R}}\times\overline{\mathbb{R}}\rightarrow \mathbb{R}:(x,y)\rightarrow|\overline{atan}(x)-\overline{atan}(y)|[/tex]
With this definition, the extended reals become a compact, connected metric space. This definition also allows us to define limits and continuity in the ordinary sense, and these limits correspond to the limits without extended reals. For example [itex]\lim_{x\rightarrow +\infty}{f(x)}=a[/itex] in the extended reals if and only if
[tex]\forall \varepsilon >0:~ \exists P:~\forall x\geq P:~|f(x)-a|<\varepsilon[/tex]
As a topological space, the extended reals are homeomorph to [0,1].
* This entry is from our old Library feature. If you know who wrote it, please let us know so we can attribute a writer. Thanks!