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[SOLVED] Compact Set Question

Sudharaka

Well-known member
MHB Math Helper
Feb 5, 2012
1,621
Hi everyone, :)

I encountered the following question recently. :)

Let \((X,\,d)\) be a metric space and \(\lim_{n\rightarrow\infty}d(x_n,\, x_0)=0\). Prove that the set \(\{x_{j}\}_{j=1}^{\infty}\) is compact.
Now I think this question is wrong. Let me give a counterexample. Take the set of real numbers with the usual Euclidean metric. Then take for example the sequence, \(\{\frac{1}{n}\}_{n=1}^{\infty}\). Then,

\[\lim_{n\rightarrow\infty}d(x_n,\, x_0)=\lim_{n\rightarrow\infty}\left|\frac{1}{n}-0\right|=0\]

All subsequences of \(\{\frac{1}{n}\}_{n=1}^{\infty}\) should converge to the same limit, which in this case is zero. Hence \(\{\frac{1}{n}\}_{n=1}^{\infty}\) is not compact as the limiting value of the sequence (and hence all subsequences) does not belong to \(\{\frac{1}{n}\}_{n=1}^{\infty}\). Let me know if I am wrong. :)

Thank you.
 

HallsofIvy

Well-known member
MHB Math Helper
Jan 29, 2012
1,151
I agree with you. However, the set [tex]\{x_j\}_{j= 0}^\infty[/tex] is compact. Could that "j= 1" be a printing error?
 

Sudharaka

Well-known member
MHB Math Helper
Feb 5, 2012
1,621
I agree with you. However, the set [tex]\{x_j\}_{j= 0}^\infty[/tex] is compact. Could that "j= 1" be a printing error?
Hi HallsofIvy, :)

Thanks for replying. But how can that make a difference? For example I can define a sequence in the real numbers like,

\[x_{0}=1\mbox{ and }x_n=\frac{1}{n}\mbox{ for }n\geq 1\]

which is again convergent to \(0\) but not compact.
 

Plato

Well-known member
MHB Math Helper
Jan 27, 2012
196
I agree with you. However, the set [tex]\{x_j\}_{j= 0}^\infty[/tex] is compact. Could that "j= 1" be a printing error?
But how can that make a difference? For example I can define a sequence in the real numbers like,
\[x_{0}=1\mbox{ and }x_n=\frac{1}{n}\mbox{ for }n\geq 1\]
which is again convergent to \(0\) but not compact.
But now [tex]{\lim _{n \to \infty }}d({a_n},{a_0}) \ne 0[/tex]
 

Sudharaka

Well-known member
MHB Math Helper
Feb 5, 2012
1,621
But how can that make a difference? For example I can define a sequence in the real numbers like,
\[x_{0}=1\mbox{ and }x_n=\frac{1}{n}\mbox{ for }n\geq 1\]
which is again convergent to \(0\) but not compact.
But now [tex]{\lim _{n \to \infty }}d({a_n},{a_0}) \ne 0[/tex]
Arghaaaaaa...... How could I missed that.... (Angry) (Headbang)

Thanks for pointing that out. :)