Is Every Convergent Sequence in a Closed Set a Cauchy Sequence?

In summary, a set F is considered closed if every Cauchy sequence contained in F has a limit that is also an element of F. This can be proven by showing that every closed set contains all of its limit points and that every convergent sequence in F is also a Cauchy sequence.
  • #1
kathrynag
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Homework Statement



A set F[tex]\subseteq[/tex]R is closed iff every Cauchy sequence contained in F has a limit that is also an element of F.

Homework Equations





The Attempt at a Solution


Let F be closed. Then F contains its limit points.
This means x=lim[tex]a_{n}[/tex] are elements of F.
 
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  • #2
I guess the formal proof is getting me:
Every closed set contains all of its limit points. Then there is a a neighborhood(x) such that the intersection of F is not equal to {x}. I guess I don't see how to get to the Cauchy sequence from there.

Other direction: Let an be cauchy sequence such that an has a limit contained in F.
an is a Cauchy sequence if abs value(an-am)<epsilon
A limit exists for an if abs value(an-a)<epsilon
we can rewrite an-am as (an-a)+(a-am)
Then by the triangle inequality, abs value(an-am)<abs value(an-a)+abs value(a-am)
I'm stuck going from there to the set being closed
 
  • #3
i think every convergent sequence in F is a cauchy sequence.
 

1. What is a Closed Cauchy sequence?

A Closed Cauchy sequence is a sequence of real numbers that satisfies the Cauchy criterion, which states that for any positive real number, there exists a point in the sequence after which all subsequent terms are within that distance from each other. This means that the terms in the sequence are getting closer and closer together as the sequence progresses.

2. What is the importance of Closed Cauchy sequences in mathematics?

Closed Cauchy sequences are important in mathematics because they provide a way to rigorously define the concept of convergence. They are used in the proof of the completeness of the real numbers, which states that every Cauchy sequence of real numbers converges to a real number.

3. How is a Closed Cauchy sequence different from other types of sequences?

A Closed Cauchy sequence is different from other types of sequences in that it satisfies the Cauchy criterion, which is a stronger condition than the usual definition of a convergent sequence. This means that every convergent sequence is also a Closed Cauchy sequence, but the converse is not necessarily true.

4. What is the proof for a Closed Cauchy sequence convergence?

The proof for the convergence of a Closed Cauchy sequence involves showing that the terms in the sequence get closer and closer together as the sequence progresses, and that there exists a limit point to which the sequence converges. This can be done using the Cauchy criterion and the completeness of the real numbers.

5. Are there any practical applications of Closed Cauchy sequences?

Yes, Closed Cauchy sequences have practical applications in various fields of mathematics, such as analysis, probability, and differential equations. They are also used in engineering and physics to model and analyze various systems and phenomena.

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