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.







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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.
    – 4-ier
    Aug 24 at 3:05














up vote
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down vote

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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.







share|cite|improve this question




















  • 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.
    – 4-ier
    Aug 24 at 3:05












up vote
1
down vote

favorite
1









up vote
1
down vote

favorite
1






1





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.







share|cite|improve this question












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.









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asked Aug 24 at 2:54









RandomWalker

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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.
    – 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.
    – 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.
– 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.
– 4-ier
Aug 24 at 3:05










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
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      active

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      2 Answers
      2






      active

      oldest

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      active

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      active

      oldest

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      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].






      share|cite|improve this answer
























        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].






        share|cite|improve this answer






















          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].






          share|cite|improve this answer












          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].







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Aug 24 at 6:16









          Kavi Rama Murthy

          23.9k31033




          23.9k31033




















              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.






              share|cite|improve this answer


























                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.






                share|cite|improve this answer
























                  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.






                  share|cite|improve this answer














                  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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                  edited Aug 25 at 0:04

























                  answered Aug 24 at 17:42









                  Mike Earnest

                  17.3k11749




                  17.3k11749



























                       

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