Convergent sequences in metric space $(mathbb{R}^n,d)$
$begingroup$
Given a metric $d : mathbb{R}^n times mathbb{R}^n mapsto mathbb{R}$ defined as
$$ d(x,y) = begin{cases}
lVert x rVert + lVert y rVert & xneq y \
0 & x = y
end{cases}
$$
How can one describe the convergent sequences in metric space $(mathbb{R}^n,d)$?
My work:
This metric will return a longer distance between any two points $x, y in mathbb{R}^n$ So by the triangle inequality if a sequence converges in $(mathbb{R}^n,d)$ it must also converge in $mathbb{R}^n$ with the standard metric $lVert x - y rVert$ but the reverse is not necessarily true, the standard metric on $mathbb{R}^n$ is finer.
What about Cauchy sequences? It's not very clear to me whether $(mathbb{R}^n,d)$ is a complete metric space.
real-analysis metric-spaces
$endgroup$
add a comment |
$begingroup$
Given a metric $d : mathbb{R}^n times mathbb{R}^n mapsto mathbb{R}$ defined as
$$ d(x,y) = begin{cases}
lVert x rVert + lVert y rVert & xneq y \
0 & x = y
end{cases}
$$
How can one describe the convergent sequences in metric space $(mathbb{R}^n,d)$?
My work:
This metric will return a longer distance between any two points $x, y in mathbb{R}^n$ So by the triangle inequality if a sequence converges in $(mathbb{R}^n,d)$ it must also converge in $mathbb{R}^n$ with the standard metric $lVert x - y rVert$ but the reverse is not necessarily true, the standard metric on $mathbb{R}^n$ is finer.
What about Cauchy sequences? It's not very clear to me whether $(mathbb{R}^n,d)$ is a complete metric space.
real-analysis metric-spaces
$endgroup$
$begingroup$
Hints: What is in a small neighbourhood of $x$ when (a) $xneq 0$ and (b) $x=0$?
$endgroup$
– Peter Franek
Dec 16 '18 at 22:07
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In case $x = 0$, it's the same epsilon neighbourhood around zero with the standard metric. If $x neq 0$ then it's an epsilon neighbourhood that completely contains the epsilon neighbourhood with the standard metric..
$endgroup$
– Sptmp
Dec 16 '18 at 22:18
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@Sptmp And note that if $xne 0$, there are lots of $epsilon>0$ for which you cant be within an $epsilon$ ball of the point, indeed $epsilon<||x||$.
$endgroup$
– qbert
Dec 16 '18 at 22:23
1
$begingroup$
(@Sptmp Indeed, everywhere apart of zero, the space is discrete.)
$endgroup$
– Peter Franek
Dec 16 '18 at 22:51
add a comment |
$begingroup$
Given a metric $d : mathbb{R}^n times mathbb{R}^n mapsto mathbb{R}$ defined as
$$ d(x,y) = begin{cases}
lVert x rVert + lVert y rVert & xneq y \
0 & x = y
end{cases}
$$
How can one describe the convergent sequences in metric space $(mathbb{R}^n,d)$?
My work:
This metric will return a longer distance between any two points $x, y in mathbb{R}^n$ So by the triangle inequality if a sequence converges in $(mathbb{R}^n,d)$ it must also converge in $mathbb{R}^n$ with the standard metric $lVert x - y rVert$ but the reverse is not necessarily true, the standard metric on $mathbb{R}^n$ is finer.
What about Cauchy sequences? It's not very clear to me whether $(mathbb{R}^n,d)$ is a complete metric space.
real-analysis metric-spaces
$endgroup$
Given a metric $d : mathbb{R}^n times mathbb{R}^n mapsto mathbb{R}$ defined as
$$ d(x,y) = begin{cases}
lVert x rVert + lVert y rVert & xneq y \
0 & x = y
end{cases}
$$
How can one describe the convergent sequences in metric space $(mathbb{R}^n,d)$?
My work:
This metric will return a longer distance between any two points $x, y in mathbb{R}^n$ So by the triangle inequality if a sequence converges in $(mathbb{R}^n,d)$ it must also converge in $mathbb{R}^n$ with the standard metric $lVert x - y rVert$ but the reverse is not necessarily true, the standard metric on $mathbb{R}^n$ is finer.
What about Cauchy sequences? It's not very clear to me whether $(mathbb{R}^n,d)$ is a complete metric space.
real-analysis metric-spaces
real-analysis metric-spaces
asked Dec 16 '18 at 22:04
SptmpSptmp
7914
7914
$begingroup$
Hints: What is in a small neighbourhood of $x$ when (a) $xneq 0$ and (b) $x=0$?
$endgroup$
– Peter Franek
Dec 16 '18 at 22:07
$begingroup$
In case $x = 0$, it's the same epsilon neighbourhood around zero with the standard metric. If $x neq 0$ then it's an epsilon neighbourhood that completely contains the epsilon neighbourhood with the standard metric..
$endgroup$
– Sptmp
Dec 16 '18 at 22:18
$begingroup$
@Sptmp And note that if $xne 0$, there are lots of $epsilon>0$ for which you cant be within an $epsilon$ ball of the point, indeed $epsilon<||x||$.
$endgroup$
– qbert
Dec 16 '18 at 22:23
1
$begingroup$
(@Sptmp Indeed, everywhere apart of zero, the space is discrete.)
$endgroup$
– Peter Franek
Dec 16 '18 at 22:51
add a comment |
$begingroup$
Hints: What is in a small neighbourhood of $x$ when (a) $xneq 0$ and (b) $x=0$?
$endgroup$
– Peter Franek
Dec 16 '18 at 22:07
$begingroup$
In case $x = 0$, it's the same epsilon neighbourhood around zero with the standard metric. If $x neq 0$ then it's an epsilon neighbourhood that completely contains the epsilon neighbourhood with the standard metric..
$endgroup$
– Sptmp
Dec 16 '18 at 22:18
$begingroup$
@Sptmp And note that if $xne 0$, there are lots of $epsilon>0$ for which you cant be within an $epsilon$ ball of the point, indeed $epsilon<||x||$.
$endgroup$
– qbert
Dec 16 '18 at 22:23
1
$begingroup$
(@Sptmp Indeed, everywhere apart of zero, the space is discrete.)
$endgroup$
– Peter Franek
Dec 16 '18 at 22:51
$begingroup$
Hints: What is in a small neighbourhood of $x$ when (a) $xneq 0$ and (b) $x=0$?
$endgroup$
– Peter Franek
Dec 16 '18 at 22:07
$begingroup$
Hints: What is in a small neighbourhood of $x$ when (a) $xneq 0$ and (b) $x=0$?
$endgroup$
– Peter Franek
Dec 16 '18 at 22:07
$begingroup$
In case $x = 0$, it's the same epsilon neighbourhood around zero with the standard metric. If $x neq 0$ then it's an epsilon neighbourhood that completely contains the epsilon neighbourhood with the standard metric..
$endgroup$
– Sptmp
Dec 16 '18 at 22:18
$begingroup$
In case $x = 0$, it's the same epsilon neighbourhood around zero with the standard metric. If $x neq 0$ then it's an epsilon neighbourhood that completely contains the epsilon neighbourhood with the standard metric..
$endgroup$
– Sptmp
Dec 16 '18 at 22:18
$begingroup$
@Sptmp And note that if $xne 0$, there are lots of $epsilon>0$ for which you cant be within an $epsilon$ ball of the point, indeed $epsilon<||x||$.
$endgroup$
– qbert
Dec 16 '18 at 22:23
$begingroup$
@Sptmp And note that if $xne 0$, there are lots of $epsilon>0$ for which you cant be within an $epsilon$ ball of the point, indeed $epsilon<||x||$.
$endgroup$
– qbert
Dec 16 '18 at 22:23
1
1
$begingroup$
(@Sptmp Indeed, everywhere apart of zero, the space is discrete.)
$endgroup$
– Peter Franek
Dec 16 '18 at 22:51
$begingroup$
(@Sptmp Indeed, everywhere apart of zero, the space is discrete.)
$endgroup$
– Peter Franek
Dec 16 '18 at 22:51
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
So assume that $(x_n)$ converges to $x$. It follows that $d(x_n, x)$ is arbitrarly small. Now assume that $(x_n)$ is not eventually constant, i.e. $x_nneq x$ for infinitely many $n$. Then $d(x_n,x)=lVert x_nrVert+lVert xrVertgeq lVert xrVert$. If $xneq 0$ then $d(x_n,x)geq lVert xrVert>0$ and thus $x_n$ cannot be convergent to $x$.
It follows that if $xneq 0$ then $(x_n)to x$ if and only if $(x_n)$ is eventually constant.
On the other hand following similar reasoning we conclude that $(x_n)to 0$ in $d$ if and only if $(x_n)to 0$ in $lVertcdotrVert$.
Similarly if $(x_n)$ is Cauchy then $d(x_n, x_m)=lVert x_nrVert +lVert x_mrVert$ assuming $(x_n)$ is not eventually constant (which is the trivial case). So $d(x_n,x_m)$ is arbitrarly small if and only if $lVert x_nrVert$ is arbitrarly small. Meaning that $(x_n)$ is Cauchy if and only if it converges to $0$ (I omit the case when $(x_n)$ is eventually constant). In particular $(mathbb{R}^n,d)$ is complete.
$endgroup$
add a comment |
$begingroup$
Note that if for any $kgeq 0$,
$$
d(x_{n},x_{n+k})to 0iff ||x_n||+||x_{n+k}||to 0implies ||x_n||to 0
$$
So, for any Cauchy sequence, we have
$$
d(x_n,0)=||x_{n}||to 0
$$
and $x_nto 0$ in this metric.
Since convergent sequences are always Cauchy, this goes a long way towards describing the convergent sequences, even in terms of the usual Euclidean metric.
$endgroup$
add a comment |
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
So assume that $(x_n)$ converges to $x$. It follows that $d(x_n, x)$ is arbitrarly small. Now assume that $(x_n)$ is not eventually constant, i.e. $x_nneq x$ for infinitely many $n$. Then $d(x_n,x)=lVert x_nrVert+lVert xrVertgeq lVert xrVert$. If $xneq 0$ then $d(x_n,x)geq lVert xrVert>0$ and thus $x_n$ cannot be convergent to $x$.
It follows that if $xneq 0$ then $(x_n)to x$ if and only if $(x_n)$ is eventually constant.
On the other hand following similar reasoning we conclude that $(x_n)to 0$ in $d$ if and only if $(x_n)to 0$ in $lVertcdotrVert$.
Similarly if $(x_n)$ is Cauchy then $d(x_n, x_m)=lVert x_nrVert +lVert x_mrVert$ assuming $(x_n)$ is not eventually constant (which is the trivial case). So $d(x_n,x_m)$ is arbitrarly small if and only if $lVert x_nrVert$ is arbitrarly small. Meaning that $(x_n)$ is Cauchy if and only if it converges to $0$ (I omit the case when $(x_n)$ is eventually constant). In particular $(mathbb{R}^n,d)$ is complete.
$endgroup$
add a comment |
$begingroup$
So assume that $(x_n)$ converges to $x$. It follows that $d(x_n, x)$ is arbitrarly small. Now assume that $(x_n)$ is not eventually constant, i.e. $x_nneq x$ for infinitely many $n$. Then $d(x_n,x)=lVert x_nrVert+lVert xrVertgeq lVert xrVert$. If $xneq 0$ then $d(x_n,x)geq lVert xrVert>0$ and thus $x_n$ cannot be convergent to $x$.
It follows that if $xneq 0$ then $(x_n)to x$ if and only if $(x_n)$ is eventually constant.
On the other hand following similar reasoning we conclude that $(x_n)to 0$ in $d$ if and only if $(x_n)to 0$ in $lVertcdotrVert$.
Similarly if $(x_n)$ is Cauchy then $d(x_n, x_m)=lVert x_nrVert +lVert x_mrVert$ assuming $(x_n)$ is not eventually constant (which is the trivial case). So $d(x_n,x_m)$ is arbitrarly small if and only if $lVert x_nrVert$ is arbitrarly small. Meaning that $(x_n)$ is Cauchy if and only if it converges to $0$ (I omit the case when $(x_n)$ is eventually constant). In particular $(mathbb{R}^n,d)$ is complete.
$endgroup$
add a comment |
$begingroup$
So assume that $(x_n)$ converges to $x$. It follows that $d(x_n, x)$ is arbitrarly small. Now assume that $(x_n)$ is not eventually constant, i.e. $x_nneq x$ for infinitely many $n$. Then $d(x_n,x)=lVert x_nrVert+lVert xrVertgeq lVert xrVert$. If $xneq 0$ then $d(x_n,x)geq lVert xrVert>0$ and thus $x_n$ cannot be convergent to $x$.
It follows that if $xneq 0$ then $(x_n)to x$ if and only if $(x_n)$ is eventually constant.
On the other hand following similar reasoning we conclude that $(x_n)to 0$ in $d$ if and only if $(x_n)to 0$ in $lVertcdotrVert$.
Similarly if $(x_n)$ is Cauchy then $d(x_n, x_m)=lVert x_nrVert +lVert x_mrVert$ assuming $(x_n)$ is not eventually constant (which is the trivial case). So $d(x_n,x_m)$ is arbitrarly small if and only if $lVert x_nrVert$ is arbitrarly small. Meaning that $(x_n)$ is Cauchy if and only if it converges to $0$ (I omit the case when $(x_n)$ is eventually constant). In particular $(mathbb{R}^n,d)$ is complete.
$endgroup$
So assume that $(x_n)$ converges to $x$. It follows that $d(x_n, x)$ is arbitrarly small. Now assume that $(x_n)$ is not eventually constant, i.e. $x_nneq x$ for infinitely many $n$. Then $d(x_n,x)=lVert x_nrVert+lVert xrVertgeq lVert xrVert$. If $xneq 0$ then $d(x_n,x)geq lVert xrVert>0$ and thus $x_n$ cannot be convergent to $x$.
It follows that if $xneq 0$ then $(x_n)to x$ if and only if $(x_n)$ is eventually constant.
On the other hand following similar reasoning we conclude that $(x_n)to 0$ in $d$ if and only if $(x_n)to 0$ in $lVertcdotrVert$.
Similarly if $(x_n)$ is Cauchy then $d(x_n, x_m)=lVert x_nrVert +lVert x_mrVert$ assuming $(x_n)$ is not eventually constant (which is the trivial case). So $d(x_n,x_m)$ is arbitrarly small if and only if $lVert x_nrVert$ is arbitrarly small. Meaning that $(x_n)$ is Cauchy if and only if it converges to $0$ (I omit the case when $(x_n)$ is eventually constant). In particular $(mathbb{R}^n,d)$ is complete.
answered Dec 16 '18 at 22:23
freakishfreakish
12.8k1631
12.8k1631
add a comment |
add a comment |
$begingroup$
Note that if for any $kgeq 0$,
$$
d(x_{n},x_{n+k})to 0iff ||x_n||+||x_{n+k}||to 0implies ||x_n||to 0
$$
So, for any Cauchy sequence, we have
$$
d(x_n,0)=||x_{n}||to 0
$$
and $x_nto 0$ in this metric.
Since convergent sequences are always Cauchy, this goes a long way towards describing the convergent sequences, even in terms of the usual Euclidean metric.
$endgroup$
add a comment |
$begingroup$
Note that if for any $kgeq 0$,
$$
d(x_{n},x_{n+k})to 0iff ||x_n||+||x_{n+k}||to 0implies ||x_n||to 0
$$
So, for any Cauchy sequence, we have
$$
d(x_n,0)=||x_{n}||to 0
$$
and $x_nto 0$ in this metric.
Since convergent sequences are always Cauchy, this goes a long way towards describing the convergent sequences, even in terms of the usual Euclidean metric.
$endgroup$
add a comment |
$begingroup$
Note that if for any $kgeq 0$,
$$
d(x_{n},x_{n+k})to 0iff ||x_n||+||x_{n+k}||to 0implies ||x_n||to 0
$$
So, for any Cauchy sequence, we have
$$
d(x_n,0)=||x_{n}||to 0
$$
and $x_nto 0$ in this metric.
Since convergent sequences are always Cauchy, this goes a long way towards describing the convergent sequences, even in terms of the usual Euclidean metric.
$endgroup$
Note that if for any $kgeq 0$,
$$
d(x_{n},x_{n+k})to 0iff ||x_n||+||x_{n+k}||to 0implies ||x_n||to 0
$$
So, for any Cauchy sequence, we have
$$
d(x_n,0)=||x_{n}||to 0
$$
and $x_nto 0$ in this metric.
Since convergent sequences are always Cauchy, this goes a long way towards describing the convergent sequences, even in terms of the usual Euclidean metric.
answered Dec 16 '18 at 22:17
qbertqbert
22.2k32561
22.2k32561
add a comment |
add a comment |
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$begingroup$
Hints: What is in a small neighbourhood of $x$ when (a) $xneq 0$ and (b) $x=0$?
$endgroup$
– Peter Franek
Dec 16 '18 at 22:07
$begingroup$
In case $x = 0$, it's the same epsilon neighbourhood around zero with the standard metric. If $x neq 0$ then it's an epsilon neighbourhood that completely contains the epsilon neighbourhood with the standard metric..
$endgroup$
– Sptmp
Dec 16 '18 at 22:18
$begingroup$
@Sptmp And note that if $xne 0$, there are lots of $epsilon>0$ for which you cant be within an $epsilon$ ball of the point, indeed $epsilon<||x||$.
$endgroup$
– qbert
Dec 16 '18 at 22:23
1
$begingroup$
(@Sptmp Indeed, everywhere apart of zero, the space is discrete.)
$endgroup$
– Peter Franek
Dec 16 '18 at 22:51