Showing this metric space is complete
$begingroup$
Let $X=(0,1]$ and $d(x,y)=left|frac{1}{x}-frac{1}{y}right|$. I've proven $(X,d)$ is a metric space but I don't know how to show its completeness. How can I do that?
metric-spaces
$endgroup$
add a comment |
$begingroup$
Let $X=(0,1]$ and $d(x,y)=left|frac{1}{x}-frac{1}{y}right|$. I've proven $(X,d)$ is a metric space but I don't know how to show its completeness. How can I do that?
metric-spaces
$endgroup$
$begingroup$
Start with a Cauchy Sequence in $(0,1]$ and show that it has a limit in $(0,1]$
$endgroup$
– crskhr
Oct 12 '14 at 8:59
$begingroup$
{1/x_n} is a Cauchy sequence but its limit 0 isn't in (0,1]
$endgroup$
– GiulyB
Oct 12 '14 at 9:01
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what is $1/x_n$???
$endgroup$
– Ittay Weiss
Oct 12 '14 at 9:03
2
$begingroup$
Assuming that your $frac1{x_n}$ is simply $frac1n$, it’s not a Cauchy sequence with respect to the metric $d$: $$dleft(frac1n,frac1mright)=|n-m|;.$$
$endgroup$
– Brian M. Scott
Oct 12 '14 at 9:03
add a comment |
$begingroup$
Let $X=(0,1]$ and $d(x,y)=left|frac{1}{x}-frac{1}{y}right|$. I've proven $(X,d)$ is a metric space but I don't know how to show its completeness. How can I do that?
metric-spaces
$endgroup$
Let $X=(0,1]$ and $d(x,y)=left|frac{1}{x}-frac{1}{y}right|$. I've proven $(X,d)$ is a metric space but I don't know how to show its completeness. How can I do that?
metric-spaces
metric-spaces
edited Oct 12 '14 at 11:01
MickG
4,35731856
4,35731856
asked Oct 12 '14 at 8:57
GiulyBGiulyB
436
436
$begingroup$
Start with a Cauchy Sequence in $(0,1]$ and show that it has a limit in $(0,1]$
$endgroup$
– crskhr
Oct 12 '14 at 8:59
$begingroup$
{1/x_n} is a Cauchy sequence but its limit 0 isn't in (0,1]
$endgroup$
– GiulyB
Oct 12 '14 at 9:01
$begingroup$
what is $1/x_n$???
$endgroup$
– Ittay Weiss
Oct 12 '14 at 9:03
2
$begingroup$
Assuming that your $frac1{x_n}$ is simply $frac1n$, it’s not a Cauchy sequence with respect to the metric $d$: $$dleft(frac1n,frac1mright)=|n-m|;.$$
$endgroup$
– Brian M. Scott
Oct 12 '14 at 9:03
add a comment |
$begingroup$
Start with a Cauchy Sequence in $(0,1]$ and show that it has a limit in $(0,1]$
$endgroup$
– crskhr
Oct 12 '14 at 8:59
$begingroup$
{1/x_n} is a Cauchy sequence but its limit 0 isn't in (0,1]
$endgroup$
– GiulyB
Oct 12 '14 at 9:01
$begingroup$
what is $1/x_n$???
$endgroup$
– Ittay Weiss
Oct 12 '14 at 9:03
2
$begingroup$
Assuming that your $frac1{x_n}$ is simply $frac1n$, it’s not a Cauchy sequence with respect to the metric $d$: $$dleft(frac1n,frac1mright)=|n-m|;.$$
$endgroup$
– Brian M. Scott
Oct 12 '14 at 9:03
$begingroup$
Start with a Cauchy Sequence in $(0,1]$ and show that it has a limit in $(0,1]$
$endgroup$
– crskhr
Oct 12 '14 at 8:59
$begingroup$
Start with a Cauchy Sequence in $(0,1]$ and show that it has a limit in $(0,1]$
$endgroup$
– crskhr
Oct 12 '14 at 8:59
$begingroup$
{1/x_n} is a Cauchy sequence but its limit 0 isn't in (0,1]
$endgroup$
– GiulyB
Oct 12 '14 at 9:01
$begingroup$
{1/x_n} is a Cauchy sequence but its limit 0 isn't in (0,1]
$endgroup$
– GiulyB
Oct 12 '14 at 9:01
$begingroup$
what is $1/x_n$???
$endgroup$
– Ittay Weiss
Oct 12 '14 at 9:03
$begingroup$
what is $1/x_n$???
$endgroup$
– Ittay Weiss
Oct 12 '14 at 9:03
2
2
$begingroup$
Assuming that your $frac1{x_n}$ is simply $frac1n$, it’s not a Cauchy sequence with respect to the metric $d$: $$dleft(frac1n,frac1mright)=|n-m|;.$$
$endgroup$
– Brian M. Scott
Oct 12 '14 at 9:03
$begingroup$
Assuming that your $frac1{x_n}$ is simply $frac1n$, it’s not a Cauchy sequence with respect to the metric $d$: $$dleft(frac1n,frac1mright)=|n-m|;.$$
$endgroup$
– Brian M. Scott
Oct 12 '14 at 9:03
add a comment |
3 Answers
3
active
oldest
votes
$begingroup$
HINT:
The map $x mapsto frac{1}{x}$ from your space to $[1, infty)$ with the usual metric is an isometry.
$endgroup$
add a comment |
$begingroup$
Expanding on orangeskid's hint-answer, "isometry" means that if we call that map $f$, the distance on $X$ $d$ and the usual metric $d_e$, then $d(x,y)=d_e(f(x),f(y))$. This can be used to turn $x_n$ into a Cauchy sequence in $[1,infty)$, thus finding a limit there and getting back to $X$.
$frac{1}{n}$ is a Cauchy sequence in $[0,1]$ or any subset of $mathbb{R}$ containing all its terms with the Euclid metric, not with your metric, where, as Brian M. Scott pointed out, the distances are never less than one.
Finally, a comment on format: it is better to provide a generic title like "Proving a metric space is complete" or "Proving a certain metric space is complete" and then putting the definition into the question rather than placing the definition of the space and distance in the title and then simply stating you can't find a proof in the question. I'll edit the question accordingly, but keep this in mind for the future :).
$endgroup$
add a comment |
$begingroup$
Let $x_n$ be a Cauchy sequence in $(X,d)$. This is equivalent to
$$forall_{epsilon>0}exists_{N in mathbb{N}}forall_{n,m>N} left|frac{1}{x_n}-frac{1}{x_m}right| < epsilon$$
But then $frac{1}{x_n} =:y_n$ is a Cauchy sequence in $(mathbb{R},p)$ with $p(x,y) = |x-y|$. Because $(mathbb{R},p)$ is complete there exists a number $x$ such that $lim_{n to infty}y_n = x$. Note that $x geq 1$, because $0<x_nleq 1$. $lim_{n to infty}y_n = x$ is again equivalent by the definition of convergence to $$forall_{epsilon>0}exists_{N in mathbb{N}}forall_{n,m>N} left|frac{1}{x_n}-xright| < epsilon$$
But this is the same as
$$forall_{epsilon>0}exists_{N in mathbb{N}}forall_{n,m>N} d(x_n,frac{1}{x}) < epsilon$$ which means that $lim_{n to infty}x_n = frac{1}{x} in X$.
$endgroup$
add a comment |
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3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
HINT:
The map $x mapsto frac{1}{x}$ from your space to $[1, infty)$ with the usual metric is an isometry.
$endgroup$
add a comment |
$begingroup$
HINT:
The map $x mapsto frac{1}{x}$ from your space to $[1, infty)$ with the usual metric is an isometry.
$endgroup$
add a comment |
$begingroup$
HINT:
The map $x mapsto frac{1}{x}$ from your space to $[1, infty)$ with the usual metric is an isometry.
$endgroup$
HINT:
The map $x mapsto frac{1}{x}$ from your space to $[1, infty)$ with the usual metric is an isometry.
answered Oct 12 '14 at 9:06
Orest BucicovschiOrest Bucicovschi
28.5k31747
28.5k31747
add a comment |
add a comment |
$begingroup$
Expanding on orangeskid's hint-answer, "isometry" means that if we call that map $f$, the distance on $X$ $d$ and the usual metric $d_e$, then $d(x,y)=d_e(f(x),f(y))$. This can be used to turn $x_n$ into a Cauchy sequence in $[1,infty)$, thus finding a limit there and getting back to $X$.
$frac{1}{n}$ is a Cauchy sequence in $[0,1]$ or any subset of $mathbb{R}$ containing all its terms with the Euclid metric, not with your metric, where, as Brian M. Scott pointed out, the distances are never less than one.
Finally, a comment on format: it is better to provide a generic title like "Proving a metric space is complete" or "Proving a certain metric space is complete" and then putting the definition into the question rather than placing the definition of the space and distance in the title and then simply stating you can't find a proof in the question. I'll edit the question accordingly, but keep this in mind for the future :).
$endgroup$
add a comment |
$begingroup$
Expanding on orangeskid's hint-answer, "isometry" means that if we call that map $f$, the distance on $X$ $d$ and the usual metric $d_e$, then $d(x,y)=d_e(f(x),f(y))$. This can be used to turn $x_n$ into a Cauchy sequence in $[1,infty)$, thus finding a limit there and getting back to $X$.
$frac{1}{n}$ is a Cauchy sequence in $[0,1]$ or any subset of $mathbb{R}$ containing all its terms with the Euclid metric, not with your metric, where, as Brian M. Scott pointed out, the distances are never less than one.
Finally, a comment on format: it is better to provide a generic title like "Proving a metric space is complete" or "Proving a certain metric space is complete" and then putting the definition into the question rather than placing the definition of the space and distance in the title and then simply stating you can't find a proof in the question. I'll edit the question accordingly, but keep this in mind for the future :).
$endgroup$
add a comment |
$begingroup$
Expanding on orangeskid's hint-answer, "isometry" means that if we call that map $f$, the distance on $X$ $d$ and the usual metric $d_e$, then $d(x,y)=d_e(f(x),f(y))$. This can be used to turn $x_n$ into a Cauchy sequence in $[1,infty)$, thus finding a limit there and getting back to $X$.
$frac{1}{n}$ is a Cauchy sequence in $[0,1]$ or any subset of $mathbb{R}$ containing all its terms with the Euclid metric, not with your metric, where, as Brian M. Scott pointed out, the distances are never less than one.
Finally, a comment on format: it is better to provide a generic title like "Proving a metric space is complete" or "Proving a certain metric space is complete" and then putting the definition into the question rather than placing the definition of the space and distance in the title and then simply stating you can't find a proof in the question. I'll edit the question accordingly, but keep this in mind for the future :).
$endgroup$
Expanding on orangeskid's hint-answer, "isometry" means that if we call that map $f$, the distance on $X$ $d$ and the usual metric $d_e$, then $d(x,y)=d_e(f(x),f(y))$. This can be used to turn $x_n$ into a Cauchy sequence in $[1,infty)$, thus finding a limit there and getting back to $X$.
$frac{1}{n}$ is a Cauchy sequence in $[0,1]$ or any subset of $mathbb{R}$ containing all its terms with the Euclid metric, not with your metric, where, as Brian M. Scott pointed out, the distances are never less than one.
Finally, a comment on format: it is better to provide a generic title like "Proving a metric space is complete" or "Proving a certain metric space is complete" and then putting the definition into the question rather than placing the definition of the space and distance in the title and then simply stating you can't find a proof in the question. I'll edit the question accordingly, but keep this in mind for the future :).
answered Oct 12 '14 at 10:47
MickGMickG
4,35731856
4,35731856
add a comment |
add a comment |
$begingroup$
Let $x_n$ be a Cauchy sequence in $(X,d)$. This is equivalent to
$$forall_{epsilon>0}exists_{N in mathbb{N}}forall_{n,m>N} left|frac{1}{x_n}-frac{1}{x_m}right| < epsilon$$
But then $frac{1}{x_n} =:y_n$ is a Cauchy sequence in $(mathbb{R},p)$ with $p(x,y) = |x-y|$. Because $(mathbb{R},p)$ is complete there exists a number $x$ such that $lim_{n to infty}y_n = x$. Note that $x geq 1$, because $0<x_nleq 1$. $lim_{n to infty}y_n = x$ is again equivalent by the definition of convergence to $$forall_{epsilon>0}exists_{N in mathbb{N}}forall_{n,m>N} left|frac{1}{x_n}-xright| < epsilon$$
But this is the same as
$$forall_{epsilon>0}exists_{N in mathbb{N}}forall_{n,m>N} d(x_n,frac{1}{x}) < epsilon$$ which means that $lim_{n to infty}x_n = frac{1}{x} in X$.
$endgroup$
add a comment |
$begingroup$
Let $x_n$ be a Cauchy sequence in $(X,d)$. This is equivalent to
$$forall_{epsilon>0}exists_{N in mathbb{N}}forall_{n,m>N} left|frac{1}{x_n}-frac{1}{x_m}right| < epsilon$$
But then $frac{1}{x_n} =:y_n$ is a Cauchy sequence in $(mathbb{R},p)$ with $p(x,y) = |x-y|$. Because $(mathbb{R},p)$ is complete there exists a number $x$ such that $lim_{n to infty}y_n = x$. Note that $x geq 1$, because $0<x_nleq 1$. $lim_{n to infty}y_n = x$ is again equivalent by the definition of convergence to $$forall_{epsilon>0}exists_{N in mathbb{N}}forall_{n,m>N} left|frac{1}{x_n}-xright| < epsilon$$
But this is the same as
$$forall_{epsilon>0}exists_{N in mathbb{N}}forall_{n,m>N} d(x_n,frac{1}{x}) < epsilon$$ which means that $lim_{n to infty}x_n = frac{1}{x} in X$.
$endgroup$
add a comment |
$begingroup$
Let $x_n$ be a Cauchy sequence in $(X,d)$. This is equivalent to
$$forall_{epsilon>0}exists_{N in mathbb{N}}forall_{n,m>N} left|frac{1}{x_n}-frac{1}{x_m}right| < epsilon$$
But then $frac{1}{x_n} =:y_n$ is a Cauchy sequence in $(mathbb{R},p)$ with $p(x,y) = |x-y|$. Because $(mathbb{R},p)$ is complete there exists a number $x$ such that $lim_{n to infty}y_n = x$. Note that $x geq 1$, because $0<x_nleq 1$. $lim_{n to infty}y_n = x$ is again equivalent by the definition of convergence to $$forall_{epsilon>0}exists_{N in mathbb{N}}forall_{n,m>N} left|frac{1}{x_n}-xright| < epsilon$$
But this is the same as
$$forall_{epsilon>0}exists_{N in mathbb{N}}forall_{n,m>N} d(x_n,frac{1}{x}) < epsilon$$ which means that $lim_{n to infty}x_n = frac{1}{x} in X$.
$endgroup$
Let $x_n$ be a Cauchy sequence in $(X,d)$. This is equivalent to
$$forall_{epsilon>0}exists_{N in mathbb{N}}forall_{n,m>N} left|frac{1}{x_n}-frac{1}{x_m}right| < epsilon$$
But then $frac{1}{x_n} =:y_n$ is a Cauchy sequence in $(mathbb{R},p)$ with $p(x,y) = |x-y|$. Because $(mathbb{R},p)$ is complete there exists a number $x$ such that $lim_{n to infty}y_n = x$. Note that $x geq 1$, because $0<x_nleq 1$. $lim_{n to infty}y_n = x$ is again equivalent by the definition of convergence to $$forall_{epsilon>0}exists_{N in mathbb{N}}forall_{n,m>N} left|frac{1}{x_n}-xright| < epsilon$$
But this is the same as
$$forall_{epsilon>0}exists_{N in mathbb{N}}forall_{n,m>N} d(x_n,frac{1}{x}) < epsilon$$ which means that $lim_{n to infty}x_n = frac{1}{x} in X$.
edited Jul 22 '15 at 9:48
answered Jul 21 '15 at 21:50
Andrei KhAndrei Kh
1,100818
1,100818
add a comment |
add a comment |
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$begingroup$
Start with a Cauchy Sequence in $(0,1]$ and show that it has a limit in $(0,1]$
$endgroup$
– crskhr
Oct 12 '14 at 8:59
$begingroup$
{1/x_n} is a Cauchy sequence but its limit 0 isn't in (0,1]
$endgroup$
– GiulyB
Oct 12 '14 at 9:01
$begingroup$
what is $1/x_n$???
$endgroup$
– Ittay Weiss
Oct 12 '14 at 9:03
2
$begingroup$
Assuming that your $frac1{x_n}$ is simply $frac1n$, it’s not a Cauchy sequence with respect to the metric $d$: $$dleft(frac1n,frac1mright)=|n-m|;.$$
$endgroup$
– Brian M. Scott
Oct 12 '14 at 9:03