A sequence in $mathbb{R}$ that has no Cauchy subsequence
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Give an example of a sequence in $mathbb{R}$ which has no subsequence which is a Cauchy sequence.
I can find out a sequence that is not a Cauchy sequence such as ${ln(n)}$ once $|ln(n)-ln(n+1)|=0$ but $|ln(n)-ln(2n)|=|ln(frac{1}{2})|>epsilon$
$forall epsilon<ln(frac{1}{2})$
I can still find a subsequence of the type ${ln(2n)}_{2ninmathbb{N}}$ such that $|ln(2n)-ln(2n+1)|=0$
Question:
What should I do to get a sequence that has no Cauchy subsequence?
Thanks in advance!
real-analysis sequences-and-series general-topology metric-spaces cauchy-sequences
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add a comment |
$begingroup$
Give an example of a sequence in $mathbb{R}$ which has no subsequence which is a Cauchy sequence.
I can find out a sequence that is not a Cauchy sequence such as ${ln(n)}$ once $|ln(n)-ln(n+1)|=0$ but $|ln(n)-ln(2n)|=|ln(frac{1}{2})|>epsilon$
$forall epsilon<ln(frac{1}{2})$
I can still find a subsequence of the type ${ln(2n)}_{2ninmathbb{N}}$ such that $|ln(2n)-ln(2n+1)|=0$
Question:
What should I do to get a sequence that has no Cauchy subsequence?
Thanks in advance!
real-analysis sequences-and-series general-topology metric-spaces cauchy-sequences
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2
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In the first place, you should only be looking at sequences which are not convergent, right?
$endgroup$
– MPW
Nov 22 '18 at 22:57
4
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I do not understand your question. What do you mean by $|ln(2n)-ln(2n+1)|=0$?
$endgroup$
– Carsten S
Nov 23 '18 at 10:29
add a comment |
$begingroup$
Give an example of a sequence in $mathbb{R}$ which has no subsequence which is a Cauchy sequence.
I can find out a sequence that is not a Cauchy sequence such as ${ln(n)}$ once $|ln(n)-ln(n+1)|=0$ but $|ln(n)-ln(2n)|=|ln(frac{1}{2})|>epsilon$
$forall epsilon<ln(frac{1}{2})$
I can still find a subsequence of the type ${ln(2n)}_{2ninmathbb{N}}$ such that $|ln(2n)-ln(2n+1)|=0$
Question:
What should I do to get a sequence that has no Cauchy subsequence?
Thanks in advance!
real-analysis sequences-and-series general-topology metric-spaces cauchy-sequences
$endgroup$
Give an example of a sequence in $mathbb{R}$ which has no subsequence which is a Cauchy sequence.
I can find out a sequence that is not a Cauchy sequence such as ${ln(n)}$ once $|ln(n)-ln(n+1)|=0$ but $|ln(n)-ln(2n)|=|ln(frac{1}{2})|>epsilon$
$forall epsilon<ln(frac{1}{2})$
I can still find a subsequence of the type ${ln(2n)}_{2ninmathbb{N}}$ such that $|ln(2n)-ln(2n+1)|=0$
Question:
What should I do to get a sequence that has no Cauchy subsequence?
Thanks in advance!
real-analysis sequences-and-series general-topology metric-spaces cauchy-sequences
real-analysis sequences-and-series general-topology metric-spaces cauchy-sequences
edited Nov 24 '18 at 7:17
user21820
39.4k543155
39.4k543155
asked Nov 22 '18 at 22:36
Pedro GomesPedro Gomes
1,9252721
1,9252721
2
$begingroup$
In the first place, you should only be looking at sequences which are not convergent, right?
$endgroup$
– MPW
Nov 22 '18 at 22:57
4
$begingroup$
I do not understand your question. What do you mean by $|ln(2n)-ln(2n+1)|=0$?
$endgroup$
– Carsten S
Nov 23 '18 at 10:29
add a comment |
2
$begingroup$
In the first place, you should only be looking at sequences which are not convergent, right?
$endgroup$
– MPW
Nov 22 '18 at 22:57
4
$begingroup$
I do not understand your question. What do you mean by $|ln(2n)-ln(2n+1)|=0$?
$endgroup$
– Carsten S
Nov 23 '18 at 10:29
2
2
$begingroup$
In the first place, you should only be looking at sequences which are not convergent, right?
$endgroup$
– MPW
Nov 22 '18 at 22:57
$begingroup$
In the first place, you should only be looking at sequences which are not convergent, right?
$endgroup$
– MPW
Nov 22 '18 at 22:57
4
4
$begingroup$
I do not understand your question. What do you mean by $|ln(2n)-ln(2n+1)|=0$?
$endgroup$
– Carsten S
Nov 23 '18 at 10:29
$begingroup$
I do not understand your question. What do you mean by $|ln(2n)-ln(2n+1)|=0$?
$endgroup$
– Carsten S
Nov 23 '18 at 10:29
add a comment |
6 Answers
6
active
oldest
votes
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Take $a_n=n$. Then for any subsequence $n_k$, $|n_k-n_{k-1}|geq 1$. So, it has no Cauchy sub-sequence.
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add a comment |
$begingroup$
Take a sequence $(a_n)$ such that $a_nto infty$. Then each subsequence $(a_{n_k})$ is such that $a_{n_k}to infty$ and thus can't be Cauchy since necessarily Cauchy sequences are bounded.
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add a comment |
$begingroup$
Take the sequence $1,2,3,4,ldots$ It has no Cauchy subsequence since the distance between any two distinct terms is at least $1$.
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add a comment |
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I didn't understand your analysis but sequence $a_n = ln n$ doesn't have Cauchy subsequence since its limit is $infty$
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I guess this one uses the theorem that every Cauchy sequence converges
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– user334732
Nov 23 '18 at 21:15
add a comment |
$begingroup$
As Foobaz John says, any sequence with $a_ntoinfty$ works, and it's easy to see that $|a_n|toinfty$ is also sufficient. In fact this latter condition is necessary and sufficient. If $|a_n|nottoinfty$, then by definition there is some $c>0$ for which there are infinitely many values $n_i$ such that $|a_{n_i}|<c$ for each $i$. Now by Bolzano-Weierstrass the subsequence $a_{n_i}$ has an infinite subsequence $a_{n_{i_j}}$ which is convergent, and therefore Cauchy.
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$begingroup$
Ah, Bolzano-Weierstrass, also known as the theorem about subsubsubscripts.
$endgroup$
– Misha Lavrov
Nov 23 '18 at 18:32
add a comment |
$begingroup$
Bolzano-Weierstrass: Every bounded sequence has a convergent subsequence.
Fact: Every convergent sequence is bounded.
Strategy: Try an unbounded sequence.
Guess: $a_n=n$
Conclusion: (I leave it to you)
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add a comment |
Your Answer
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6 Answers
6
active
oldest
votes
6 Answers
6
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
Take $a_n=n$. Then for any subsequence $n_k$, $|n_k-n_{k-1}|geq 1$. So, it has no Cauchy sub-sequence.
$endgroup$
add a comment |
$begingroup$
Take $a_n=n$. Then for any subsequence $n_k$, $|n_k-n_{k-1}|geq 1$. So, it has no Cauchy sub-sequence.
$endgroup$
add a comment |
$begingroup$
Take $a_n=n$. Then for any subsequence $n_k$, $|n_k-n_{k-1}|geq 1$. So, it has no Cauchy sub-sequence.
$endgroup$
Take $a_n=n$. Then for any subsequence $n_k$, $|n_k-n_{k-1}|geq 1$. So, it has no Cauchy sub-sequence.
answered Nov 22 '18 at 22:40
John_WickJohn_Wick
1,616111
1,616111
add a comment |
add a comment |
$begingroup$
Take a sequence $(a_n)$ such that $a_nto infty$. Then each subsequence $(a_{n_k})$ is such that $a_{n_k}to infty$ and thus can't be Cauchy since necessarily Cauchy sequences are bounded.
$endgroup$
add a comment |
$begingroup$
Take a sequence $(a_n)$ such that $a_nto infty$. Then each subsequence $(a_{n_k})$ is such that $a_{n_k}to infty$ and thus can't be Cauchy since necessarily Cauchy sequences are bounded.
$endgroup$
add a comment |
$begingroup$
Take a sequence $(a_n)$ such that $a_nto infty$. Then each subsequence $(a_{n_k})$ is such that $a_{n_k}to infty$ and thus can't be Cauchy since necessarily Cauchy sequences are bounded.
$endgroup$
Take a sequence $(a_n)$ such that $a_nto infty$. Then each subsequence $(a_{n_k})$ is such that $a_{n_k}to infty$ and thus can't be Cauchy since necessarily Cauchy sequences are bounded.
answered Nov 22 '18 at 22:56
Foobaz JohnFoobaz John
22.7k41452
22.7k41452
add a comment |
add a comment |
$begingroup$
Take the sequence $1,2,3,4,ldots$ It has no Cauchy subsequence since the distance between any two distinct terms is at least $1$.
$endgroup$
add a comment |
$begingroup$
Take the sequence $1,2,3,4,ldots$ It has no Cauchy subsequence since the distance between any two distinct terms is at least $1$.
$endgroup$
add a comment |
$begingroup$
Take the sequence $1,2,3,4,ldots$ It has no Cauchy subsequence since the distance between any two distinct terms is at least $1$.
$endgroup$
Take the sequence $1,2,3,4,ldots$ It has no Cauchy subsequence since the distance between any two distinct terms is at least $1$.
answered Nov 22 '18 at 22:39
José Carlos SantosJosé Carlos Santos
168k22132236
168k22132236
add a comment |
add a comment |
$begingroup$
I didn't understand your analysis but sequence $a_n = ln n$ doesn't have Cauchy subsequence since its limit is $infty$
$endgroup$
$begingroup$
I guess this one uses the theorem that every Cauchy sequence converges
$endgroup$
– user334732
Nov 23 '18 at 21:15
add a comment |
$begingroup$
I didn't understand your analysis but sequence $a_n = ln n$ doesn't have Cauchy subsequence since its limit is $infty$
$endgroup$
$begingroup$
I guess this one uses the theorem that every Cauchy sequence converges
$endgroup$
– user334732
Nov 23 '18 at 21:15
add a comment |
$begingroup$
I didn't understand your analysis but sequence $a_n = ln n$ doesn't have Cauchy subsequence since its limit is $infty$
$endgroup$
I didn't understand your analysis but sequence $a_n = ln n$ doesn't have Cauchy subsequence since its limit is $infty$
answered Nov 23 '18 at 8:09
RiaDRiaD
725718
725718
$begingroup$
I guess this one uses the theorem that every Cauchy sequence converges
$endgroup$
– user334732
Nov 23 '18 at 21:15
add a comment |
$begingroup$
I guess this one uses the theorem that every Cauchy sequence converges
$endgroup$
– user334732
Nov 23 '18 at 21:15
$begingroup$
I guess this one uses the theorem that every Cauchy sequence converges
$endgroup$
– user334732
Nov 23 '18 at 21:15
$begingroup$
I guess this one uses the theorem that every Cauchy sequence converges
$endgroup$
– user334732
Nov 23 '18 at 21:15
add a comment |
$begingroup$
As Foobaz John says, any sequence with $a_ntoinfty$ works, and it's easy to see that $|a_n|toinfty$ is also sufficient. In fact this latter condition is necessary and sufficient. If $|a_n|nottoinfty$, then by definition there is some $c>0$ for which there are infinitely many values $n_i$ such that $|a_{n_i}|<c$ for each $i$. Now by Bolzano-Weierstrass the subsequence $a_{n_i}$ has an infinite subsequence $a_{n_{i_j}}$ which is convergent, and therefore Cauchy.
$endgroup$
$begingroup$
Ah, Bolzano-Weierstrass, also known as the theorem about subsubsubscripts.
$endgroup$
– Misha Lavrov
Nov 23 '18 at 18:32
add a comment |
$begingroup$
As Foobaz John says, any sequence with $a_ntoinfty$ works, and it's easy to see that $|a_n|toinfty$ is also sufficient. In fact this latter condition is necessary and sufficient. If $|a_n|nottoinfty$, then by definition there is some $c>0$ for which there are infinitely many values $n_i$ such that $|a_{n_i}|<c$ for each $i$. Now by Bolzano-Weierstrass the subsequence $a_{n_i}$ has an infinite subsequence $a_{n_{i_j}}$ which is convergent, and therefore Cauchy.
$endgroup$
$begingroup$
Ah, Bolzano-Weierstrass, also known as the theorem about subsubsubscripts.
$endgroup$
– Misha Lavrov
Nov 23 '18 at 18:32
add a comment |
$begingroup$
As Foobaz John says, any sequence with $a_ntoinfty$ works, and it's easy to see that $|a_n|toinfty$ is also sufficient. In fact this latter condition is necessary and sufficient. If $|a_n|nottoinfty$, then by definition there is some $c>0$ for which there are infinitely many values $n_i$ such that $|a_{n_i}|<c$ for each $i$. Now by Bolzano-Weierstrass the subsequence $a_{n_i}$ has an infinite subsequence $a_{n_{i_j}}$ which is convergent, and therefore Cauchy.
$endgroup$
As Foobaz John says, any sequence with $a_ntoinfty$ works, and it's easy to see that $|a_n|toinfty$ is also sufficient. In fact this latter condition is necessary and sufficient. If $|a_n|nottoinfty$, then by definition there is some $c>0$ for which there are infinitely many values $n_i$ such that $|a_{n_i}|<c$ for each $i$. Now by Bolzano-Weierstrass the subsequence $a_{n_i}$ has an infinite subsequence $a_{n_{i_j}}$ which is convergent, and therefore Cauchy.
answered Nov 23 '18 at 13:02
Especially LimeEspecially Lime
22.4k22858
22.4k22858
$begingroup$
Ah, Bolzano-Weierstrass, also known as the theorem about subsubsubscripts.
$endgroup$
– Misha Lavrov
Nov 23 '18 at 18:32
add a comment |
$begingroup$
Ah, Bolzano-Weierstrass, also known as the theorem about subsubsubscripts.
$endgroup$
– Misha Lavrov
Nov 23 '18 at 18:32
$begingroup$
Ah, Bolzano-Weierstrass, also known as the theorem about subsubsubscripts.
$endgroup$
– Misha Lavrov
Nov 23 '18 at 18:32
$begingroup$
Ah, Bolzano-Weierstrass, also known as the theorem about subsubsubscripts.
$endgroup$
– Misha Lavrov
Nov 23 '18 at 18:32
add a comment |
$begingroup$
Bolzano-Weierstrass: Every bounded sequence has a convergent subsequence.
Fact: Every convergent sequence is bounded.
Strategy: Try an unbounded sequence.
Guess: $a_n=n$
Conclusion: (I leave it to you)
$endgroup$
add a comment |
$begingroup$
Bolzano-Weierstrass: Every bounded sequence has a convergent subsequence.
Fact: Every convergent sequence is bounded.
Strategy: Try an unbounded sequence.
Guess: $a_n=n$
Conclusion: (I leave it to you)
$endgroup$
add a comment |
$begingroup$
Bolzano-Weierstrass: Every bounded sequence has a convergent subsequence.
Fact: Every convergent sequence is bounded.
Strategy: Try an unbounded sequence.
Guess: $a_n=n$
Conclusion: (I leave it to you)
$endgroup$
Bolzano-Weierstrass: Every bounded sequence has a convergent subsequence.
Fact: Every convergent sequence is bounded.
Strategy: Try an unbounded sequence.
Guess: $a_n=n$
Conclusion: (I leave it to you)
answered Nov 24 '18 at 3:26
user198044
add a comment |
add a comment |
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2
$begingroup$
In the first place, you should only be looking at sequences which are not convergent, right?
$endgroup$
– MPW
Nov 22 '18 at 22:57
4
$begingroup$
I do not understand your question. What do you mean by $|ln(2n)-ln(2n+1)|=0$?
$endgroup$
– Carsten S
Nov 23 '18 at 10:29