Let $f: X rightarrow X$ be a homeomorphism of a compact metric space. If the orbit of $x$ is compact, then $x$ is periodic
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Let $f: X rightarrow X$ be a homeomorphism of a compact, connected metric space. If the orbit of $x$ is compact, then $x$ is periodic.
I feel like should be trivial, but I cannot seem to work out a proof.
Since $f(Orb(x)) = Orb(x)$ and $Orb(x)$ is minimal, it follows that $Orb(x)$ is nowhere dense.
real-analysis dynamical-systems
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Let $f: X rightarrow X$ be a homeomorphism of a compact, connected metric space. If the orbit of $x$ is compact, then $x$ is periodic.
I feel like should be trivial, but I cannot seem to work out a proof.
Since $f(Orb(x)) = Orb(x)$ and $Orb(x)$ is minimal, it follows that $Orb(x)$ is nowhere dense.
real-analysis dynamical-systems
What is the definition of the orbit? Is it include applying $f^-1$ as well? If not, then I don't see that the the image of the orbit under $f$ is itself, unless the orbit is in fact periodic.
â 4-ier
Aug 24 at 3:05
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up vote
1
down vote
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up vote
1
down vote
favorite
Let $f: X rightarrow X$ be a homeomorphism of a compact, connected metric space. If the orbit of $x$ is compact, then $x$ is periodic.
I feel like should be trivial, but I cannot seem to work out a proof.
Since $f(Orb(x)) = Orb(x)$ and $Orb(x)$ is minimal, it follows that $Orb(x)$ is nowhere dense.
real-analysis dynamical-systems
Let $f: X rightarrow X$ be a homeomorphism of a compact, connected metric space. If the orbit of $x$ is compact, then $x$ is periodic.
I feel like should be trivial, but I cannot seem to work out a proof.
Since $f(Orb(x)) = Orb(x)$ and $Orb(x)$ is minimal, it follows that $Orb(x)$ is nowhere dense.
real-analysis dynamical-systems
asked Aug 24 at 2:54
RandomWalker
1299
1299
What is the definition of the orbit? Is it include applying $f^-1$ as well? If not, then I don't see that the the image of the orbit under $f$ is itself, unless the orbit is in fact periodic.
â 4-ier
Aug 24 at 3:05
add a comment |Â
What is the definition of the orbit? Is it include applying $f^-1$ as well? If not, then I don't see that the the image of the orbit under $f$ is itself, unless the orbit is in fact periodic.
â 4-ier
Aug 24 at 3:05
What is the definition of the orbit? Is it include applying $f^-1$ as well? If not, then I don't see that the the image of the orbit under $f$ is itself, unless the orbit is in fact periodic.
â 4-ier
Aug 24 at 3:05
What is the definition of the orbit? Is it include applying $f^-1$ as well? If not, then I don't see that the the image of the orbit under $f$ is itself, unless the orbit is in fact periodic.
â 4-ier
Aug 24 at 3:05
add a comment |Â
2 Answers
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Assuming that the orbits are two-sided orbits consider the orbit $Y$ of $x$. This is a countable compact metric space and $f$ is a homeomorphism of $Y$ onto itself. Write $Y$ as the union of its singletons and apply BCT to conclude that some point in $Y$ is open. This implies that each point of $Y$ is open because $f$ is a homeomorphism. Compactness of $Y$ implies that $Y$ is a finite set. [Look at the open cover formed by singletons].
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Kavi Rama Murthy's proof can be modified to work when the orbit $O=x_0,f(x_0),f(f(x_0)),dots$ is one sided. As in their proof, some point $x$ in $O$ is open in $O$. This means there is an open $Usubset X$ for which $Ucap O=x$. This implies $f(x)$ is open as well, as
$$
f(x)=f(Ucap O)=f(U) cap f(O)=(f(U)setminus x_0)cap O
$$
Note $f(Acap B)=f(A)cap f(B)$ since $f$ is injective.
Supposing $x$ is not the initial point $x_0$, then
beginalign
f^-1(x)
&=f^-1(Ucap O)
\&=f^-1(U)cap f^-1(O)
\&=f^-1(U)cap (Ocup f^-1(x_0))
\&=(f^-1(U)cap O)cup f^-1(Ucap x_0)
\&=f^-1(U)cap O
endalign
The last equality follows since $Ucap O=x$, so $x_0notin U$. This shows $f^-1(x)$ is open.
Therefore, the fact $x$ is open implies all points in the orbit are open, and you can conclude as in the other proof.
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
6
down vote
accepted
Assuming that the orbits are two-sided orbits consider the orbit $Y$ of $x$. This is a countable compact metric space and $f$ is a homeomorphism of $Y$ onto itself. Write $Y$ as the union of its singletons and apply BCT to conclude that some point in $Y$ is open. This implies that each point of $Y$ is open because $f$ is a homeomorphism. Compactness of $Y$ implies that $Y$ is a finite set. [Look at the open cover formed by singletons].
add a comment |Â
up vote
6
down vote
accepted
Assuming that the orbits are two-sided orbits consider the orbit $Y$ of $x$. This is a countable compact metric space and $f$ is a homeomorphism of $Y$ onto itself. Write $Y$ as the union of its singletons and apply BCT to conclude that some point in $Y$ is open. This implies that each point of $Y$ is open because $f$ is a homeomorphism. Compactness of $Y$ implies that $Y$ is a finite set. [Look at the open cover formed by singletons].
add a comment |Â
up vote
6
down vote
accepted
up vote
6
down vote
accepted
Assuming that the orbits are two-sided orbits consider the orbit $Y$ of $x$. This is a countable compact metric space and $f$ is a homeomorphism of $Y$ onto itself. Write $Y$ as the union of its singletons and apply BCT to conclude that some point in $Y$ is open. This implies that each point of $Y$ is open because $f$ is a homeomorphism. Compactness of $Y$ implies that $Y$ is a finite set. [Look at the open cover formed by singletons].
Assuming that the orbits are two-sided orbits consider the orbit $Y$ of $x$. This is a countable compact metric space and $f$ is a homeomorphism of $Y$ onto itself. Write $Y$ as the union of its singletons and apply BCT to conclude that some point in $Y$ is open. This implies that each point of $Y$ is open because $f$ is a homeomorphism. Compactness of $Y$ implies that $Y$ is a finite set. [Look at the open cover formed by singletons].
answered Aug 24 at 6:16
Kavi Rama Murthy
23.9k31033
23.9k31033
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up vote
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Kavi Rama Murthy's proof can be modified to work when the orbit $O=x_0,f(x_0),f(f(x_0)),dots$ is one sided. As in their proof, some point $x$ in $O$ is open in $O$. This means there is an open $Usubset X$ for which $Ucap O=x$. This implies $f(x)$ is open as well, as
$$
f(x)=f(Ucap O)=f(U) cap f(O)=(f(U)setminus x_0)cap O
$$
Note $f(Acap B)=f(A)cap f(B)$ since $f$ is injective.
Supposing $x$ is not the initial point $x_0$, then
beginalign
f^-1(x)
&=f^-1(Ucap O)
\&=f^-1(U)cap f^-1(O)
\&=f^-1(U)cap (Ocup f^-1(x_0))
\&=(f^-1(U)cap O)cup f^-1(Ucap x_0)
\&=f^-1(U)cap O
endalign
The last equality follows since $Ucap O=x$, so $x_0notin U$. This shows $f^-1(x)$ is open.
Therefore, the fact $x$ is open implies all points in the orbit are open, and you can conclude as in the other proof.
add a comment |Â
up vote
4
down vote
Kavi Rama Murthy's proof can be modified to work when the orbit $O=x_0,f(x_0),f(f(x_0)),dots$ is one sided. As in their proof, some point $x$ in $O$ is open in $O$. This means there is an open $Usubset X$ for which $Ucap O=x$. This implies $f(x)$ is open as well, as
$$
f(x)=f(Ucap O)=f(U) cap f(O)=(f(U)setminus x_0)cap O
$$
Note $f(Acap B)=f(A)cap f(B)$ since $f$ is injective.
Supposing $x$ is not the initial point $x_0$, then
beginalign
f^-1(x)
&=f^-1(Ucap O)
\&=f^-1(U)cap f^-1(O)
\&=f^-1(U)cap (Ocup f^-1(x_0))
\&=(f^-1(U)cap O)cup f^-1(Ucap x_0)
\&=f^-1(U)cap O
endalign
The last equality follows since $Ucap O=x$, so $x_0notin U$. This shows $f^-1(x)$ is open.
Therefore, the fact $x$ is open implies all points in the orbit are open, and you can conclude as in the other proof.
add a comment |Â
up vote
4
down vote
up vote
4
down vote
Kavi Rama Murthy's proof can be modified to work when the orbit $O=x_0,f(x_0),f(f(x_0)),dots$ is one sided. As in their proof, some point $x$ in $O$ is open in $O$. This means there is an open $Usubset X$ for which $Ucap O=x$. This implies $f(x)$ is open as well, as
$$
f(x)=f(Ucap O)=f(U) cap f(O)=(f(U)setminus x_0)cap O
$$
Note $f(Acap B)=f(A)cap f(B)$ since $f$ is injective.
Supposing $x$ is not the initial point $x_0$, then
beginalign
f^-1(x)
&=f^-1(Ucap O)
\&=f^-1(U)cap f^-1(O)
\&=f^-1(U)cap (Ocup f^-1(x_0))
\&=(f^-1(U)cap O)cup f^-1(Ucap x_0)
\&=f^-1(U)cap O
endalign
The last equality follows since $Ucap O=x$, so $x_0notin U$. This shows $f^-1(x)$ is open.
Therefore, the fact $x$ is open implies all points in the orbit are open, and you can conclude as in the other proof.
Kavi Rama Murthy's proof can be modified to work when the orbit $O=x_0,f(x_0),f(f(x_0)),dots$ is one sided. As in their proof, some point $x$ in $O$ is open in $O$. This means there is an open $Usubset X$ for which $Ucap O=x$. This implies $f(x)$ is open as well, as
$$
f(x)=f(Ucap O)=f(U) cap f(O)=(f(U)setminus x_0)cap O
$$
Note $f(Acap B)=f(A)cap f(B)$ since $f$ is injective.
Supposing $x$ is not the initial point $x_0$, then
beginalign
f^-1(x)
&=f^-1(Ucap O)
\&=f^-1(U)cap f^-1(O)
\&=f^-1(U)cap (Ocup f^-1(x_0))
\&=(f^-1(U)cap O)cup f^-1(Ucap x_0)
\&=f^-1(U)cap O
endalign
The last equality follows since $Ucap O=x$, so $x_0notin U$. This shows $f^-1(x)$ is open.
Therefore, the fact $x$ is open implies all points in the orbit are open, and you can conclude as in the other proof.
edited Aug 25 at 0:04
answered Aug 24 at 17:42
Mike Earnest
17.3k11749
17.3k11749
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What is the definition of the orbit? Is it include applying $f^-1$ as well? If not, then I don't see that the the image of the orbit under $f$ is itself, unless the orbit is in fact periodic.
â 4-ier
Aug 24 at 3:05