Cantor's finite intersection principle

In summary, the proof in the video lecture seems to be saying that if for any subcollection of a set, the intersection of all of those subcollection is not empty, then the set itself is not empty.
  • #1
Rasalhague
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I'm trying to understand the proof given in the last 10 minutes or so of this video lecture, but after some struggle, it occurs to me that I may be misinterpreting what the theorem says. According to this, Cantor's finite intersection principle states the following.

Given a metric space [itex](X,d)[/itex] and a collection of compact subsets

[tex]\left \{ K_\alpha \subseteq X \; \bigg| \; \alpha \in A \right \}[/tex]

(where [itex]A[/itex] is an index set), if the elements of any finite subcollection of [itex]\left \{ K_\alpha \right \}_{\alpha \in A}[/itex] have a nonempty intersection, then the intersection of all the [itex]K_\alpha[/itex] is nonempty.

But should "any" be read in the specific sense, "there exists", here:

[tex]\left [ \exists \left \{ K_{\alpha_i} \right \}_{i=1}^n \subseteq \left \{ K_\alpha \right \}_{\alpha \in A} \left \left ( \bigcap_{i=1}^{n} K_{\alpha_i} \neq \varnothing \right ) \right ] \Rightarrow \left [ \bigcap_{\alpha \in A} K_\alpha \neq \varnothing \right ] \enspace ?[/tex]

Or should "any" be read in the nonspecific sense, "for all":

[tex]\left [ \forall \left \{ K_{\alpha_i} \right \}_{i=1}^n \subseteq \left \{ K_\alpha \right \}_{\alpha \in A} \left \left ( \bigcap_{i=1}^{n} K_{\alpha_i} \neq \varnothing \right ) \right ] \Rightarrow \left [ \bigcap_{\alpha \in A} K_\alpha \neq \varnothing \right ] \enspace ?[/tex]

And what sense or senses could collection have here: set, class, family, multiset?
 
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  • #2
Rasalhague said:
[tex]\left [ \forall \left \{ K_{\alpha_i} \right \}_{i=1}^n \subseteq \left \{ K_\alpha \right \}_{\alpha \in A} \left \left ( \bigcap_{i=1}^{n} K_{\alpha_i} \neq \varnothing \right ) \right ] \Rightarrow \left [ \bigcap_{\alpha \in A} K_\alpha \neq \varnothing \right ] \enspace [/tex]

It's this,

[tex] \forall \left \{ K_{\alpha_i} \right \}_{i=1}^n \subseteq \left \{ K_\alpha \right \}_{\alpha \in A} \left \left ( \bigcap_{i=1}^{n} K_{\alpha_i} \neq \varnothing \right ) \Rightarrow\bigcap_{\alpha \in A} K_\alpha \neq \varnothing \enspace [/tex]

"For any", "given any" and "for all" are terms used interchangeably. "Collection" means "set" here.
 
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  • #3
Thanks very much, Jarle! No wonder I was getting confused; I'd been taking it in the other sense.
 

FAQ: Cantor's finite intersection principle

What is Cantor's finite intersection principle?

Cantor's finite intersection principle, also known as the Nested Interval Theorem, states that if you have a sequence of nested and non-empty closed intervals, then the intersection of all these intervals will also be non-empty.

What is the significance of Cantor's finite intersection principle?

Cantor's finite intersection principle is a fundamental theorem in analysis and is used to prove other important theorems, such as the Bolzano-Weierstrass theorem and the Intermediate Value Theorem. It also has applications in fields such as topology and measure theory.

How is Cantor's finite intersection principle different from the Bolzano-Weierstrass theorem?

While both theorems involve sequences of nested intervals, Cantor's finite intersection principle deals with the intersection of all the intervals in the sequence, while the Bolzano-Weierstrass theorem deals with the existence of a convergent subsequence within the sequence of intervals.

What is an example of a proof using Cantor's finite intersection principle?

One example is the proof of the Intermediate Value Theorem, which states that if a continuous function has values of opposite signs at two points, then it must have a root between those points. This theorem can be proved by constructing a sequence of nested intervals using the function and then applying Cantor's finite intersection principle to show the existence of a root.

Can Cantor's finite intersection principle be extended to infinite intersections?

No, Cantor's finite intersection principle only holds for finite intersections. If we try to extend it to infinite intersections, we may end up with an empty set as the intersection, which violates the principle.

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