Difference between revisions of "Limsup and liminf (sequence of sets)"
From Maths
(Created page with "{{Stub page}} __TOC__ ==Definition== Given a set {{M|X}} and a sequence {{M|1=(A_n)^\infty_{n=1} }} of subsets of {{M|X}}, so {{M|1=(A_n)^\infty_{n=1}\subse...") |
m |
||
| (One intermediate revision by the same user not shown) | |||
| Line 1: | Line 1: | ||
| − | {{Stub page}} | + | {{Stub page|Requires further expansion, are limsup and liminf always defined? What does limit of a sequence of sets mean?|grade=A}} |
__TOC__ | __TOC__ | ||
==Definition== | ==Definition== | ||
| Line 19: | Line 19: | ||
==References== | ==References== | ||
<references/> | <references/> | ||
| − | {{Definition|Set Theory}} | + | {{Definition|Set Theory|Measure Theory}} |
Latest revision as of 22:17, 19 April 2016
Stub grade: A
This page is a stub
This page is a stub, so it contains little or minimal information and is on a to-do list for being expanded.The message provided is:
Requires further expansion, are limsup and liminf always defined? What does limit of a sequence of sets mean?
Definition
Given a set [ilmath]X[/ilmath] and a sequence [ilmath](A_n)^\infty_{n=1}[/ilmath] of subsets of [ilmath]X[/ilmath], so [ilmath](A_n)^\infty_{n=1}\subseteq\mathcal{P}(X)[/ilmath], we may define the superior limit ([ilmath]\text{Lim sup} [/ilmath]) and inferior limit ([ilmath]\text{Lim inf} [/ilmath]) of [ilmath](A_n)[/ilmath] as follows[1]:
Lim sup
- [math]\mathop{\text{Lim sup} }_{n\rightarrow\infty}(A_n):=\Big\{x\in X\ \Big\vert\vert\{n\in\mathbb{N}\ \vert\ x\in A_n\}\vert=\aleph_0 \Big\}[/math][1]
- In words: The superior limit of [ilmath](A_n)[/ilmath] is the set that contains [ilmath]x\in X[/ilmath] given that [ilmath]x[/ilmath] is in (countably) infinitely many elements of the sequence.
Lim inf
- [math]\mathop{\text{Lim inf} }_{n\rightarrow\infty}(A_n):=\Big\{x\in X\ \Big\vert\vert\{n\in\mathbb{N}\ \vert x\notin A_n\}\vert\ne\aleph_0\Big\}[/math][1]
- In words: The inferior limit of [ilmath](A_n)[/ilmath] is the set that contains [ilmath]x\in X[/ilmath] given that [ilmath]x[/ilmath] is in all but a finite number of elements of [ilmath](A_n)[/ilmath].
Distinction
One may think to "not be in a finite number of elements" is "to be in an infinite number of elements" and conclude wrongly that these definitions are the same. This is because an element can both be in and not be in an infinite number of elements!
Example
Let [ilmath]a\in X[/ilmath] be some arbitrary point.
- Consider the sequence [ilmath](a_n)_{n=1}^\infty[/ilmath] with [ilmath]A_n:=\left\{\begin{array}{lr}\{a\} & n\text{ is even}\\\emptyset & n\text{ is odd}\end{array}\right.[/ilmath]
- Now [ilmath]a[/ilmath] is in an infinite number (namely all the even [ilmath]n[/ilmath]s) and not in an infinite number (all the odd [ilmath]n[/ilmath]s) too.
- [math]\mathop{\text{Lim sup} }_{n\rightarrow\infty}(A_n)=\{a\}[/math] as the number of elements containing [ilmath]a[/ilmath] is (countably) infinite.
- [math]\mathop{\text{Lim inf} }_{n\rightarrow\infty}(A_n)=\emptyset[/math] as no [ilmath]x\in X[/ilmath] is in an infinite number and only not in a finite number.
- Now [ilmath]a[/ilmath] is in an infinite number (namely all the even [ilmath]n[/ilmath]s) and not in an infinite number (all the odd [ilmath]n[/ilmath]s) too.
