Prove topological equivalence between two distances
Let $d(x,y)=|x-y|$ and $h(x,y)=|x-y|^{frac{1}{3}}$ be two distances on $mathbb{R}$.
I want to prove that they are topologically equivalent
I supposed that $f(x)=x$ and $g(x)=x^{frac{1}{3}}$
so we can say that
$f(x)leq g(x)$ $forall xin [0,1]$ and $f(x)geq g(x)$ $forall xgeq 1$
Will this information lead me somewhere ? If yes, how would I proceed ?
general-topology
New contributor
add a comment |
Let $d(x,y)=|x-y|$ and $h(x,y)=|x-y|^{frac{1}{3}}$ be two distances on $mathbb{R}$.
I want to prove that they are topologically equivalent
I supposed that $f(x)=x$ and $g(x)=x^{frac{1}{3}}$
so we can say that
$f(x)leq g(x)$ $forall xin [0,1]$ and $f(x)geq g(x)$ $forall xgeq 1$
Will this information lead me somewhere ? If yes, how would I proceed ?
general-topology
New contributor
add a comment |
Let $d(x,y)=|x-y|$ and $h(x,y)=|x-y|^{frac{1}{3}}$ be two distances on $mathbb{R}$.
I want to prove that they are topologically equivalent
I supposed that $f(x)=x$ and $g(x)=x^{frac{1}{3}}$
so we can say that
$f(x)leq g(x)$ $forall xin [0,1]$ and $f(x)geq g(x)$ $forall xgeq 1$
Will this information lead me somewhere ? If yes, how would I proceed ?
general-topology
New contributor
Let $d(x,y)=|x-y|$ and $h(x,y)=|x-y|^{frac{1}{3}}$ be two distances on $mathbb{R}$.
I want to prove that they are topologically equivalent
I supposed that $f(x)=x$ and $g(x)=x^{frac{1}{3}}$
so we can say that
$f(x)leq g(x)$ $forall xin [0,1]$ and $f(x)geq g(x)$ $forall xgeq 1$
Will this information lead me somewhere ? If yes, how would I proceed ?
general-topology
general-topology
New contributor
New contributor
New contributor
asked 2 days ago
Any BanyAny Bany
31
31
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New contributor
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2 Answers
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Here, I use the term "$d$-ball" to indicate a set of the form $bigl{yinBbb R:d(x,y)<rbigr}$--specifically, I'd call that the $d$-ball centered at $x$ with radius $r$--and similarly use the term "$h$-ball" to indicate a set of the form $bigl{yinBbb R:h(x,y)<rbigr}.$
To show topological equivalence, we want to show the following:
- Given any $d$-ball $B$ and any $yin B,$ there exists some $h$-ball $B'$ such that $yin B'subseteq B.$
- Given any $h$-ball $B$ and any $yin B,$ there exists some $d$-ball $B'$ such that $yin B'subseteq B.$
Your first observation makes one direction fairly easy. Pick an arbitrary $d$-ball $B,$ with center $x$ and radius $r,$ and take any $yin B.$ Find a radius $r'$ such that the $d$-ball centered at $y$ with radius $r_0$ is a subset of $B.$ Call this $d$-ball $B_0.$ Now, using your first observation, show that by choosing a radius $r'$ such that $0<r'<1$ and $r'le r_0,$ then the $h$-ball centered at $y$ of radius $r'$ is then a subset of $B_0$ containing $y,$ so you've proved 1.
To prove 2, we proceed similarly, but have more work to do. Start with an arbitrary $h$-ball $B,$ with center $x$ and radius $r,$ and take any $yin B.$ Your goal is to find some $r'>0$ such that whenever $d(y,z)<r',$ we have $h(x,z)<r.$ In other words, we need $|x-z|^{frac13}<r$ whenever $|y-z|<r'.$
By triangle inequality, we have $$|x-z|^{frac13}le |x-y|^{frac13}+|y-z|^{frac13}$$ right away, and recalling our choice of $y,$ we have that $|x-y|^{frac13}<r,$ so that $0<r-|x-y|^{frac13}.$ Consequently, if we can ensure that $|y-z|^{frac13}<r-|x-y|^{frac13},$ we'll have $|x-z|^{frac13}<r$ as desired. Can you take it from there?
1
If we choose $r'=(r-|x-y|)^{frac{1}{3}}$ it will be done right ?
– Any Bany
2 days ago
Exactly! Nicely done.
– Cameron Buie
2 days ago
You said 0<r'<1 , it should be r'>1
– Any Bany
2 days ago
Oh, dear! I did things backward. I'll edit accordingly.
– Cameron Buie
2 days ago
So now it would be $r'=(r-|x-y|^{frac{1}{3}})^3$ right ? Thanks btw
– Any Bany
2 days ago
|
show 1 more comment
Observe that for any sequences $x_n$ and $y_n$,
$$|x_n - y_n| to 0 iff |x_n - y_n|^{1/3} to 0$$
as $ntoinfty$.
Can't we prove that $exists alpha ,beta $ such that $alpha d(x,y)leq h(x,y)leq beta d(x,y)$
– Any Bany
2 days ago
For $|x| < 1$, $|x| le |x|^{1/3}$. You cannot get a constant for the reverse inequality.
– ncmathsadist
2 days ago
add a comment |
Your Answer
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2 Answers
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2 Answers
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Here, I use the term "$d$-ball" to indicate a set of the form $bigl{yinBbb R:d(x,y)<rbigr}$--specifically, I'd call that the $d$-ball centered at $x$ with radius $r$--and similarly use the term "$h$-ball" to indicate a set of the form $bigl{yinBbb R:h(x,y)<rbigr}.$
To show topological equivalence, we want to show the following:
- Given any $d$-ball $B$ and any $yin B,$ there exists some $h$-ball $B'$ such that $yin B'subseteq B.$
- Given any $h$-ball $B$ and any $yin B,$ there exists some $d$-ball $B'$ such that $yin B'subseteq B.$
Your first observation makes one direction fairly easy. Pick an arbitrary $d$-ball $B,$ with center $x$ and radius $r,$ and take any $yin B.$ Find a radius $r'$ such that the $d$-ball centered at $y$ with radius $r_0$ is a subset of $B.$ Call this $d$-ball $B_0.$ Now, using your first observation, show that by choosing a radius $r'$ such that $0<r'<1$ and $r'le r_0,$ then the $h$-ball centered at $y$ of radius $r'$ is then a subset of $B_0$ containing $y,$ so you've proved 1.
To prove 2, we proceed similarly, but have more work to do. Start with an arbitrary $h$-ball $B,$ with center $x$ and radius $r,$ and take any $yin B.$ Your goal is to find some $r'>0$ such that whenever $d(y,z)<r',$ we have $h(x,z)<r.$ In other words, we need $|x-z|^{frac13}<r$ whenever $|y-z|<r'.$
By triangle inequality, we have $$|x-z|^{frac13}le |x-y|^{frac13}+|y-z|^{frac13}$$ right away, and recalling our choice of $y,$ we have that $|x-y|^{frac13}<r,$ so that $0<r-|x-y|^{frac13}.$ Consequently, if we can ensure that $|y-z|^{frac13}<r-|x-y|^{frac13},$ we'll have $|x-z|^{frac13}<r$ as desired. Can you take it from there?
1
If we choose $r'=(r-|x-y|)^{frac{1}{3}}$ it will be done right ?
– Any Bany
2 days ago
Exactly! Nicely done.
– Cameron Buie
2 days ago
You said 0<r'<1 , it should be r'>1
– Any Bany
2 days ago
Oh, dear! I did things backward. I'll edit accordingly.
– Cameron Buie
2 days ago
So now it would be $r'=(r-|x-y|^{frac{1}{3}})^3$ right ? Thanks btw
– Any Bany
2 days ago
|
show 1 more comment
Here, I use the term "$d$-ball" to indicate a set of the form $bigl{yinBbb R:d(x,y)<rbigr}$--specifically, I'd call that the $d$-ball centered at $x$ with radius $r$--and similarly use the term "$h$-ball" to indicate a set of the form $bigl{yinBbb R:h(x,y)<rbigr}.$
To show topological equivalence, we want to show the following:
- Given any $d$-ball $B$ and any $yin B,$ there exists some $h$-ball $B'$ such that $yin B'subseteq B.$
- Given any $h$-ball $B$ and any $yin B,$ there exists some $d$-ball $B'$ such that $yin B'subseteq B.$
Your first observation makes one direction fairly easy. Pick an arbitrary $d$-ball $B,$ with center $x$ and radius $r,$ and take any $yin B.$ Find a radius $r'$ such that the $d$-ball centered at $y$ with radius $r_0$ is a subset of $B.$ Call this $d$-ball $B_0.$ Now, using your first observation, show that by choosing a radius $r'$ such that $0<r'<1$ and $r'le r_0,$ then the $h$-ball centered at $y$ of radius $r'$ is then a subset of $B_0$ containing $y,$ so you've proved 1.
To prove 2, we proceed similarly, but have more work to do. Start with an arbitrary $h$-ball $B,$ with center $x$ and radius $r,$ and take any $yin B.$ Your goal is to find some $r'>0$ such that whenever $d(y,z)<r',$ we have $h(x,z)<r.$ In other words, we need $|x-z|^{frac13}<r$ whenever $|y-z|<r'.$
By triangle inequality, we have $$|x-z|^{frac13}le |x-y|^{frac13}+|y-z|^{frac13}$$ right away, and recalling our choice of $y,$ we have that $|x-y|^{frac13}<r,$ so that $0<r-|x-y|^{frac13}.$ Consequently, if we can ensure that $|y-z|^{frac13}<r-|x-y|^{frac13},$ we'll have $|x-z|^{frac13}<r$ as desired. Can you take it from there?
1
If we choose $r'=(r-|x-y|)^{frac{1}{3}}$ it will be done right ?
– Any Bany
2 days ago
Exactly! Nicely done.
– Cameron Buie
2 days ago
You said 0<r'<1 , it should be r'>1
– Any Bany
2 days ago
Oh, dear! I did things backward. I'll edit accordingly.
– Cameron Buie
2 days ago
So now it would be $r'=(r-|x-y|^{frac{1}{3}})^3$ right ? Thanks btw
– Any Bany
2 days ago
|
show 1 more comment
Here, I use the term "$d$-ball" to indicate a set of the form $bigl{yinBbb R:d(x,y)<rbigr}$--specifically, I'd call that the $d$-ball centered at $x$ with radius $r$--and similarly use the term "$h$-ball" to indicate a set of the form $bigl{yinBbb R:h(x,y)<rbigr}.$
To show topological equivalence, we want to show the following:
- Given any $d$-ball $B$ and any $yin B,$ there exists some $h$-ball $B'$ such that $yin B'subseteq B.$
- Given any $h$-ball $B$ and any $yin B,$ there exists some $d$-ball $B'$ such that $yin B'subseteq B.$
Your first observation makes one direction fairly easy. Pick an arbitrary $d$-ball $B,$ with center $x$ and radius $r,$ and take any $yin B.$ Find a radius $r'$ such that the $d$-ball centered at $y$ with radius $r_0$ is a subset of $B.$ Call this $d$-ball $B_0.$ Now, using your first observation, show that by choosing a radius $r'$ such that $0<r'<1$ and $r'le r_0,$ then the $h$-ball centered at $y$ of radius $r'$ is then a subset of $B_0$ containing $y,$ so you've proved 1.
To prove 2, we proceed similarly, but have more work to do. Start with an arbitrary $h$-ball $B,$ with center $x$ and radius $r,$ and take any $yin B.$ Your goal is to find some $r'>0$ such that whenever $d(y,z)<r',$ we have $h(x,z)<r.$ In other words, we need $|x-z|^{frac13}<r$ whenever $|y-z|<r'.$
By triangle inequality, we have $$|x-z|^{frac13}le |x-y|^{frac13}+|y-z|^{frac13}$$ right away, and recalling our choice of $y,$ we have that $|x-y|^{frac13}<r,$ so that $0<r-|x-y|^{frac13}.$ Consequently, if we can ensure that $|y-z|^{frac13}<r-|x-y|^{frac13},$ we'll have $|x-z|^{frac13}<r$ as desired. Can you take it from there?
Here, I use the term "$d$-ball" to indicate a set of the form $bigl{yinBbb R:d(x,y)<rbigr}$--specifically, I'd call that the $d$-ball centered at $x$ with radius $r$--and similarly use the term "$h$-ball" to indicate a set of the form $bigl{yinBbb R:h(x,y)<rbigr}.$
To show topological equivalence, we want to show the following:
- Given any $d$-ball $B$ and any $yin B,$ there exists some $h$-ball $B'$ such that $yin B'subseteq B.$
- Given any $h$-ball $B$ and any $yin B,$ there exists some $d$-ball $B'$ such that $yin B'subseteq B.$
Your first observation makes one direction fairly easy. Pick an arbitrary $d$-ball $B,$ with center $x$ and radius $r,$ and take any $yin B.$ Find a radius $r'$ such that the $d$-ball centered at $y$ with radius $r_0$ is a subset of $B.$ Call this $d$-ball $B_0.$ Now, using your first observation, show that by choosing a radius $r'$ such that $0<r'<1$ and $r'le r_0,$ then the $h$-ball centered at $y$ of radius $r'$ is then a subset of $B_0$ containing $y,$ so you've proved 1.
To prove 2, we proceed similarly, but have more work to do. Start with an arbitrary $h$-ball $B,$ with center $x$ and radius $r,$ and take any $yin B.$ Your goal is to find some $r'>0$ such that whenever $d(y,z)<r',$ we have $h(x,z)<r.$ In other words, we need $|x-z|^{frac13}<r$ whenever $|y-z|<r'.$
By triangle inequality, we have $$|x-z|^{frac13}le |x-y|^{frac13}+|y-z|^{frac13}$$ right away, and recalling our choice of $y,$ we have that $|x-y|^{frac13}<r,$ so that $0<r-|x-y|^{frac13}.$ Consequently, if we can ensure that $|y-z|^{frac13}<r-|x-y|^{frac13},$ we'll have $|x-z|^{frac13}<r$ as desired. Can you take it from there?
edited 2 days ago
answered 2 days ago
Cameron BuieCameron Buie
85.1k771155
85.1k771155
1
If we choose $r'=(r-|x-y|)^{frac{1}{3}}$ it will be done right ?
– Any Bany
2 days ago
Exactly! Nicely done.
– Cameron Buie
2 days ago
You said 0<r'<1 , it should be r'>1
– Any Bany
2 days ago
Oh, dear! I did things backward. I'll edit accordingly.
– Cameron Buie
2 days ago
So now it would be $r'=(r-|x-y|^{frac{1}{3}})^3$ right ? Thanks btw
– Any Bany
2 days ago
|
show 1 more comment
1
If we choose $r'=(r-|x-y|)^{frac{1}{3}}$ it will be done right ?
– Any Bany
2 days ago
Exactly! Nicely done.
– Cameron Buie
2 days ago
You said 0<r'<1 , it should be r'>1
– Any Bany
2 days ago
Oh, dear! I did things backward. I'll edit accordingly.
– Cameron Buie
2 days ago
So now it would be $r'=(r-|x-y|^{frac{1}{3}})^3$ right ? Thanks btw
– Any Bany
2 days ago
1
1
If we choose $r'=(r-|x-y|)^{frac{1}{3}}$ it will be done right ?
– Any Bany
2 days ago
If we choose $r'=(r-|x-y|)^{frac{1}{3}}$ it will be done right ?
– Any Bany
2 days ago
Exactly! Nicely done.
– Cameron Buie
2 days ago
Exactly! Nicely done.
– Cameron Buie
2 days ago
You said 0<r'<1 , it should be r'>1
– Any Bany
2 days ago
You said 0<r'<1 , it should be r'>1
– Any Bany
2 days ago
Oh, dear! I did things backward. I'll edit accordingly.
– Cameron Buie
2 days ago
Oh, dear! I did things backward. I'll edit accordingly.
– Cameron Buie
2 days ago
So now it would be $r'=(r-|x-y|^{frac{1}{3}})^3$ right ? Thanks btw
– Any Bany
2 days ago
So now it would be $r'=(r-|x-y|^{frac{1}{3}})^3$ right ? Thanks btw
– Any Bany
2 days ago
|
show 1 more comment
Observe that for any sequences $x_n$ and $y_n$,
$$|x_n - y_n| to 0 iff |x_n - y_n|^{1/3} to 0$$
as $ntoinfty$.
Can't we prove that $exists alpha ,beta $ such that $alpha d(x,y)leq h(x,y)leq beta d(x,y)$
– Any Bany
2 days ago
For $|x| < 1$, $|x| le |x|^{1/3}$. You cannot get a constant for the reverse inequality.
– ncmathsadist
2 days ago
add a comment |
Observe that for any sequences $x_n$ and $y_n$,
$$|x_n - y_n| to 0 iff |x_n - y_n|^{1/3} to 0$$
as $ntoinfty$.
Can't we prove that $exists alpha ,beta $ such that $alpha d(x,y)leq h(x,y)leq beta d(x,y)$
– Any Bany
2 days ago
For $|x| < 1$, $|x| le |x|^{1/3}$. You cannot get a constant for the reverse inequality.
– ncmathsadist
2 days ago
add a comment |
Observe that for any sequences $x_n$ and $y_n$,
$$|x_n - y_n| to 0 iff |x_n - y_n|^{1/3} to 0$$
as $ntoinfty$.
Observe that for any sequences $x_n$ and $y_n$,
$$|x_n - y_n| to 0 iff |x_n - y_n|^{1/3} to 0$$
as $ntoinfty$.
answered 2 days ago
ncmathsadistncmathsadist
42.4k259102
42.4k259102
Can't we prove that $exists alpha ,beta $ such that $alpha d(x,y)leq h(x,y)leq beta d(x,y)$
– Any Bany
2 days ago
For $|x| < 1$, $|x| le |x|^{1/3}$. You cannot get a constant for the reverse inequality.
– ncmathsadist
2 days ago
add a comment |
Can't we prove that $exists alpha ,beta $ such that $alpha d(x,y)leq h(x,y)leq beta d(x,y)$
– Any Bany
2 days ago
For $|x| < 1$, $|x| le |x|^{1/3}$. You cannot get a constant for the reverse inequality.
– ncmathsadist
2 days ago
Can't we prove that $exists alpha ,beta $ such that $alpha d(x,y)leq h(x,y)leq beta d(x,y)$
– Any Bany
2 days ago
Can't we prove that $exists alpha ,beta $ such that $alpha d(x,y)leq h(x,y)leq beta d(x,y)$
– Any Bany
2 days ago
For $|x| < 1$, $|x| le |x|^{1/3}$. You cannot get a constant for the reverse inequality.
– ncmathsadist
2 days ago
For $|x| < 1$, $|x| le |x|^{1/3}$. You cannot get a constant for the reverse inequality.
– ncmathsadist
2 days ago
add a comment |
Any Bany is a new contributor. Be nice, and check out our Code of Conduct.
Any Bany is a new contributor. Be nice, and check out our Code of Conduct.
Any Bany is a new contributor. Be nice, and check out our Code of Conduct.
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