The smallest compactification for continuous extension of $sin(x)$












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I would like to ask what the smallest Hausdorff compactification of $mathbb{R}$ is, onto which we may continuously extend the function $g: mathbb{R} rightarrow mathbb{R}, x mapsto text{sin}(x)$.



A weaker / related version of this question has been asked here, and even that only has a partial answer: Smallest compactification for continuous extension of $sin(x)$



There the answer is about minimal compactifications; I would like to know about a smallest one. I believe, mimicking the answer given there, that the compactification I am looking for is $h=(f,g): mathbb{R} rightarrow [-1,1] times [-1,1], x mapsto (text{tanh}(x),text{sin}(x)) $.
(The first component is just an embedding of $mathbb{R}$ into an interval.) Let's call this compactification $(h,tilde{mathbb{R}})$. It is my candidate for being a smallest compactification onto which we may extend $g(x)=$ sin$(x)$.



SUMMARY: I would like to show that given any compactification $(j, gamma mathbb{R})$ such that the function $g(x)=$ sin$(x)$ extends continuously onto it, there exists a continuous map $k$ from $gamma mathbb{R}$ to $tilde{mathbb{R}}$ such that $kj=h$.



Idea for proof: I'm sure it suffices (and is necessary!) to have that $f(x)= $ tanh$(x)$ extends continuously to $(j, gamma mathbb{R})$, since by assumption $g(x)$ already extends continuously to it. However the idea I had for constructing an extension of $f(x)$ involves using sequential compactness rather than compactness, which is not allowed because in general the two notions don't coincide. For instance, I've tried to define the extension $tilde{f}: gamma mathbb{R} rightarrow [-1,1]$ as being equal to $f$ on the image of $j$ (obviously!), but then the remaining points of $gamma mathbb{R}$ have to go to either $-1$ or $1$ depending on some condition that I've not worked out. I tried to say, let $tilde{f}$ take the value $1$ (respectively $-1$) on $p in gamma mathbb{R} backslash j(mathbb{R})$ if there exists a sequence of distinct real numbers $(x_n)$ such that cos$(x_n)$ tends to $1$ (respectively $-1$) and $j(x_n)$ tends to $p$, but that's not well-defined for every $p$.










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    I would like to ask what the smallest Hausdorff compactification of $mathbb{R}$ is, onto which we may continuously extend the function $g: mathbb{R} rightarrow mathbb{R}, x mapsto text{sin}(x)$.



    A weaker / related version of this question has been asked here, and even that only has a partial answer: Smallest compactification for continuous extension of $sin(x)$



    There the answer is about minimal compactifications; I would like to know about a smallest one. I believe, mimicking the answer given there, that the compactification I am looking for is $h=(f,g): mathbb{R} rightarrow [-1,1] times [-1,1], x mapsto (text{tanh}(x),text{sin}(x)) $.
    (The first component is just an embedding of $mathbb{R}$ into an interval.) Let's call this compactification $(h,tilde{mathbb{R}})$. It is my candidate for being a smallest compactification onto which we may extend $g(x)=$ sin$(x)$.



    SUMMARY: I would like to show that given any compactification $(j, gamma mathbb{R})$ such that the function $g(x)=$ sin$(x)$ extends continuously onto it, there exists a continuous map $k$ from $gamma mathbb{R}$ to $tilde{mathbb{R}}$ such that $kj=h$.



    Idea for proof: I'm sure it suffices (and is necessary!) to have that $f(x)= $ tanh$(x)$ extends continuously to $(j, gamma mathbb{R})$, since by assumption $g(x)$ already extends continuously to it. However the idea I had for constructing an extension of $f(x)$ involves using sequential compactness rather than compactness, which is not allowed because in general the two notions don't coincide. For instance, I've tried to define the extension $tilde{f}: gamma mathbb{R} rightarrow [-1,1]$ as being equal to $f$ on the image of $j$ (obviously!), but then the remaining points of $gamma mathbb{R}$ have to go to either $-1$ or $1$ depending on some condition that I've not worked out. I tried to say, let $tilde{f}$ take the value $1$ (respectively $-1$) on $p in gamma mathbb{R} backslash j(mathbb{R})$ if there exists a sequence of distinct real numbers $(x_n)$ such that cos$(x_n)$ tends to $1$ (respectively $-1$) and $j(x_n)$ tends to $p$, but that's not well-defined for every $p$.










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      0








      0







      I would like to ask what the smallest Hausdorff compactification of $mathbb{R}$ is, onto which we may continuously extend the function $g: mathbb{R} rightarrow mathbb{R}, x mapsto text{sin}(x)$.



      A weaker / related version of this question has been asked here, and even that only has a partial answer: Smallest compactification for continuous extension of $sin(x)$



      There the answer is about minimal compactifications; I would like to know about a smallest one. I believe, mimicking the answer given there, that the compactification I am looking for is $h=(f,g): mathbb{R} rightarrow [-1,1] times [-1,1], x mapsto (text{tanh}(x),text{sin}(x)) $.
      (The first component is just an embedding of $mathbb{R}$ into an interval.) Let's call this compactification $(h,tilde{mathbb{R}})$. It is my candidate for being a smallest compactification onto which we may extend $g(x)=$ sin$(x)$.



      SUMMARY: I would like to show that given any compactification $(j, gamma mathbb{R})$ such that the function $g(x)=$ sin$(x)$ extends continuously onto it, there exists a continuous map $k$ from $gamma mathbb{R}$ to $tilde{mathbb{R}}$ such that $kj=h$.



      Idea for proof: I'm sure it suffices (and is necessary!) to have that $f(x)= $ tanh$(x)$ extends continuously to $(j, gamma mathbb{R})$, since by assumption $g(x)$ already extends continuously to it. However the idea I had for constructing an extension of $f(x)$ involves using sequential compactness rather than compactness, which is not allowed because in general the two notions don't coincide. For instance, I've tried to define the extension $tilde{f}: gamma mathbb{R} rightarrow [-1,1]$ as being equal to $f$ on the image of $j$ (obviously!), but then the remaining points of $gamma mathbb{R}$ have to go to either $-1$ or $1$ depending on some condition that I've not worked out. I tried to say, let $tilde{f}$ take the value $1$ (respectively $-1$) on $p in gamma mathbb{R} backslash j(mathbb{R})$ if there exists a sequence of distinct real numbers $(x_n)$ such that cos$(x_n)$ tends to $1$ (respectively $-1$) and $j(x_n)$ tends to $p$, but that's not well-defined for every $p$.










      share|cite|improve this question















      I would like to ask what the smallest Hausdorff compactification of $mathbb{R}$ is, onto which we may continuously extend the function $g: mathbb{R} rightarrow mathbb{R}, x mapsto text{sin}(x)$.



      A weaker / related version of this question has been asked here, and even that only has a partial answer: Smallest compactification for continuous extension of $sin(x)$



      There the answer is about minimal compactifications; I would like to know about a smallest one. I believe, mimicking the answer given there, that the compactification I am looking for is $h=(f,g): mathbb{R} rightarrow [-1,1] times [-1,1], x mapsto (text{tanh}(x),text{sin}(x)) $.
      (The first component is just an embedding of $mathbb{R}$ into an interval.) Let's call this compactification $(h,tilde{mathbb{R}})$. It is my candidate for being a smallest compactification onto which we may extend $g(x)=$ sin$(x)$.



      SUMMARY: I would like to show that given any compactification $(j, gamma mathbb{R})$ such that the function $g(x)=$ sin$(x)$ extends continuously onto it, there exists a continuous map $k$ from $gamma mathbb{R}$ to $tilde{mathbb{R}}$ such that $kj=h$.



      Idea for proof: I'm sure it suffices (and is necessary!) to have that $f(x)= $ tanh$(x)$ extends continuously to $(j, gamma mathbb{R})$, since by assumption $g(x)$ already extends continuously to it. However the idea I had for constructing an extension of $f(x)$ involves using sequential compactness rather than compactness, which is not allowed because in general the two notions don't coincide. For instance, I've tried to define the extension $tilde{f}: gamma mathbb{R} rightarrow [-1,1]$ as being equal to $f$ on the image of $j$ (obviously!), but then the remaining points of $gamma mathbb{R}$ have to go to either $-1$ or $1$ depending on some condition that I've not worked out. I tried to say, let $tilde{f}$ take the value $1$ (respectively $-1$) on $p in gamma mathbb{R} backslash j(mathbb{R})$ if there exists a sequence of distinct real numbers $(x_n)$ such that cos$(x_n)$ tends to $1$ (respectively $-1$) and $j(x_n)$ tends to $p$, but that's not well-defined for every $p$.







      general-topology






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      edited Dec 9 at 7:46









      user1101010

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      7571630










      asked Dec 9 at 6:53









      SSF

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      362110






















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          Consider the injective map $jcolonBbb Rto Bbb R^3$, $tmapsto (sin t,cos t, sin sqrt 2t)$ and let $gamma Bbb Rsubset Bbb R^2$ be the closure of the image of $j$. Clearly, $gamma Bbb R$ is the cylinder $S^1times [-1,1]$. Then $gamma Bbb Rto Bbb R$, $(x,y,z)mapsto x$ is a continuous extension of the sine function, but we cannot map the cylinder suitably to $tilde{Bbb R}$ because the points $j(2npi)$ and $j(-2npi)$ are both dense in ${(0,1)}times [-1,1]$






          share|cite|improve this answer























          • Why does the fact that those points are dense prove the cylinder can't map suitably to $tilde{mathbb{R}}$?
            – SSF
            Dec 9 at 11:32












          • Is this argument correct: If such a suitable mapping $k$ exists, then $k$ has to map that strip ${(0,1)} times [-1,1]$ to a curve in $tilde{mathbb{R}}$ on which the points $kj(2npi)=h(2npi)$, $kj(-2npi)=h(-2npi)$ are dense. But no such curve exists... (why)?
            – SSF
            Dec 9 at 11:38












          • Also, what you wrote shows that my guess of a smallest compactification was incorrect, but we still don't know that a smallest one doesn't exist, right?
            – SSF
            Dec 9 at 13:48










          • @SSF There is no theorem to tell us that a minimal such compactification exists, no. I believe the set of compactifications of a Tychonoff space $X$ always forms a poset for which sups exist. And $beta X$ always is maximal. There is a minimal compactification iff $X$ is locally compact. Beyond this not much is known.
            – Henno Brandsma
            Dec 9 at 15:32






          • 1




            See these notes for some more theory. Warning: exercise E2 is wrong...
            – Henno Brandsma
            Dec 9 at 15:45











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          Consider the injective map $jcolonBbb Rto Bbb R^3$, $tmapsto (sin t,cos t, sin sqrt 2t)$ and let $gamma Bbb Rsubset Bbb R^2$ be the closure of the image of $j$. Clearly, $gamma Bbb R$ is the cylinder $S^1times [-1,1]$. Then $gamma Bbb Rto Bbb R$, $(x,y,z)mapsto x$ is a continuous extension of the sine function, but we cannot map the cylinder suitably to $tilde{Bbb R}$ because the points $j(2npi)$ and $j(-2npi)$ are both dense in ${(0,1)}times [-1,1]$






          share|cite|improve this answer























          • Why does the fact that those points are dense prove the cylinder can't map suitably to $tilde{mathbb{R}}$?
            – SSF
            Dec 9 at 11:32












          • Is this argument correct: If such a suitable mapping $k$ exists, then $k$ has to map that strip ${(0,1)} times [-1,1]$ to a curve in $tilde{mathbb{R}}$ on which the points $kj(2npi)=h(2npi)$, $kj(-2npi)=h(-2npi)$ are dense. But no such curve exists... (why)?
            – SSF
            Dec 9 at 11:38












          • Also, what you wrote shows that my guess of a smallest compactification was incorrect, but we still don't know that a smallest one doesn't exist, right?
            – SSF
            Dec 9 at 13:48










          • @SSF There is no theorem to tell us that a minimal such compactification exists, no. I believe the set of compactifications of a Tychonoff space $X$ always forms a poset for which sups exist. And $beta X$ always is maximal. There is a minimal compactification iff $X$ is locally compact. Beyond this not much is known.
            – Henno Brandsma
            Dec 9 at 15:32






          • 1




            See these notes for some more theory. Warning: exercise E2 is wrong...
            – Henno Brandsma
            Dec 9 at 15:45
















          1














          Consider the injective map $jcolonBbb Rto Bbb R^3$, $tmapsto (sin t,cos t, sin sqrt 2t)$ and let $gamma Bbb Rsubset Bbb R^2$ be the closure of the image of $j$. Clearly, $gamma Bbb R$ is the cylinder $S^1times [-1,1]$. Then $gamma Bbb Rto Bbb R$, $(x,y,z)mapsto x$ is a continuous extension of the sine function, but we cannot map the cylinder suitably to $tilde{Bbb R}$ because the points $j(2npi)$ and $j(-2npi)$ are both dense in ${(0,1)}times [-1,1]$






          share|cite|improve this answer























          • Why does the fact that those points are dense prove the cylinder can't map suitably to $tilde{mathbb{R}}$?
            – SSF
            Dec 9 at 11:32












          • Is this argument correct: If such a suitable mapping $k$ exists, then $k$ has to map that strip ${(0,1)} times [-1,1]$ to a curve in $tilde{mathbb{R}}$ on which the points $kj(2npi)=h(2npi)$, $kj(-2npi)=h(-2npi)$ are dense. But no such curve exists... (why)?
            – SSF
            Dec 9 at 11:38












          • Also, what you wrote shows that my guess of a smallest compactification was incorrect, but we still don't know that a smallest one doesn't exist, right?
            – SSF
            Dec 9 at 13:48










          • @SSF There is no theorem to tell us that a minimal such compactification exists, no. I believe the set of compactifications of a Tychonoff space $X$ always forms a poset for which sups exist. And $beta X$ always is maximal. There is a minimal compactification iff $X$ is locally compact. Beyond this not much is known.
            – Henno Brandsma
            Dec 9 at 15:32






          • 1




            See these notes for some more theory. Warning: exercise E2 is wrong...
            – Henno Brandsma
            Dec 9 at 15:45














          1












          1








          1






          Consider the injective map $jcolonBbb Rto Bbb R^3$, $tmapsto (sin t,cos t, sin sqrt 2t)$ and let $gamma Bbb Rsubset Bbb R^2$ be the closure of the image of $j$. Clearly, $gamma Bbb R$ is the cylinder $S^1times [-1,1]$. Then $gamma Bbb Rto Bbb R$, $(x,y,z)mapsto x$ is a continuous extension of the sine function, but we cannot map the cylinder suitably to $tilde{Bbb R}$ because the points $j(2npi)$ and $j(-2npi)$ are both dense in ${(0,1)}times [-1,1]$






          share|cite|improve this answer














          Consider the injective map $jcolonBbb Rto Bbb R^3$, $tmapsto (sin t,cos t, sin sqrt 2t)$ and let $gamma Bbb Rsubset Bbb R^2$ be the closure of the image of $j$. Clearly, $gamma Bbb R$ is the cylinder $S^1times [-1,1]$. Then $gamma Bbb Rto Bbb R$, $(x,y,z)mapsto x$ is a continuous extension of the sine function, but we cannot map the cylinder suitably to $tilde{Bbb R}$ because the points $j(2npi)$ and $j(-2npi)$ are both dense in ${(0,1)}times [-1,1]$







          share|cite|improve this answer














          share|cite|improve this answer



          share|cite|improve this answer








          edited Dec 9 at 8:02









          Henno Brandsma

          105k346113




          105k346113










          answered Dec 9 at 7:39









          Hagen von Eitzen

          276k21268495




          276k21268495












          • Why does the fact that those points are dense prove the cylinder can't map suitably to $tilde{mathbb{R}}$?
            – SSF
            Dec 9 at 11:32












          • Is this argument correct: If such a suitable mapping $k$ exists, then $k$ has to map that strip ${(0,1)} times [-1,1]$ to a curve in $tilde{mathbb{R}}$ on which the points $kj(2npi)=h(2npi)$, $kj(-2npi)=h(-2npi)$ are dense. But no such curve exists... (why)?
            – SSF
            Dec 9 at 11:38












          • Also, what you wrote shows that my guess of a smallest compactification was incorrect, but we still don't know that a smallest one doesn't exist, right?
            – SSF
            Dec 9 at 13:48










          • @SSF There is no theorem to tell us that a minimal such compactification exists, no. I believe the set of compactifications of a Tychonoff space $X$ always forms a poset for which sups exist. And $beta X$ always is maximal. There is a minimal compactification iff $X$ is locally compact. Beyond this not much is known.
            – Henno Brandsma
            Dec 9 at 15:32






          • 1




            See these notes for some more theory. Warning: exercise E2 is wrong...
            – Henno Brandsma
            Dec 9 at 15:45


















          • Why does the fact that those points are dense prove the cylinder can't map suitably to $tilde{mathbb{R}}$?
            – SSF
            Dec 9 at 11:32












          • Is this argument correct: If such a suitable mapping $k$ exists, then $k$ has to map that strip ${(0,1)} times [-1,1]$ to a curve in $tilde{mathbb{R}}$ on which the points $kj(2npi)=h(2npi)$, $kj(-2npi)=h(-2npi)$ are dense. But no such curve exists... (why)?
            – SSF
            Dec 9 at 11:38












          • Also, what you wrote shows that my guess of a smallest compactification was incorrect, but we still don't know that a smallest one doesn't exist, right?
            – SSF
            Dec 9 at 13:48










          • @SSF There is no theorem to tell us that a minimal such compactification exists, no. I believe the set of compactifications of a Tychonoff space $X$ always forms a poset for which sups exist. And $beta X$ always is maximal. There is a minimal compactification iff $X$ is locally compact. Beyond this not much is known.
            – Henno Brandsma
            Dec 9 at 15:32






          • 1




            See these notes for some more theory. Warning: exercise E2 is wrong...
            – Henno Brandsma
            Dec 9 at 15:45
















          Why does the fact that those points are dense prove the cylinder can't map suitably to $tilde{mathbb{R}}$?
          – SSF
          Dec 9 at 11:32






          Why does the fact that those points are dense prove the cylinder can't map suitably to $tilde{mathbb{R}}$?
          – SSF
          Dec 9 at 11:32














          Is this argument correct: If such a suitable mapping $k$ exists, then $k$ has to map that strip ${(0,1)} times [-1,1]$ to a curve in $tilde{mathbb{R}}$ on which the points $kj(2npi)=h(2npi)$, $kj(-2npi)=h(-2npi)$ are dense. But no such curve exists... (why)?
          – SSF
          Dec 9 at 11:38






          Is this argument correct: If such a suitable mapping $k$ exists, then $k$ has to map that strip ${(0,1)} times [-1,1]$ to a curve in $tilde{mathbb{R}}$ on which the points $kj(2npi)=h(2npi)$, $kj(-2npi)=h(-2npi)$ are dense. But no such curve exists... (why)?
          – SSF
          Dec 9 at 11:38














          Also, what you wrote shows that my guess of a smallest compactification was incorrect, but we still don't know that a smallest one doesn't exist, right?
          – SSF
          Dec 9 at 13:48




          Also, what you wrote shows that my guess of a smallest compactification was incorrect, but we still don't know that a smallest one doesn't exist, right?
          – SSF
          Dec 9 at 13:48












          @SSF There is no theorem to tell us that a minimal such compactification exists, no. I believe the set of compactifications of a Tychonoff space $X$ always forms a poset for which sups exist. And $beta X$ always is maximal. There is a minimal compactification iff $X$ is locally compact. Beyond this not much is known.
          – Henno Brandsma
          Dec 9 at 15:32




          @SSF There is no theorem to tell us that a minimal such compactification exists, no. I believe the set of compactifications of a Tychonoff space $X$ always forms a poset for which sups exist. And $beta X$ always is maximal. There is a minimal compactification iff $X$ is locally compact. Beyond this not much is known.
          – Henno Brandsma
          Dec 9 at 15:32




          1




          1




          See these notes for some more theory. Warning: exercise E2 is wrong...
          – Henno Brandsma
          Dec 9 at 15:45




          See these notes for some more theory. Warning: exercise E2 is wrong...
          – Henno Brandsma
          Dec 9 at 15:45


















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