Can the surface of a 3-dimension cube be diffeomorphic to a 3-dimension sphere?












0














We all know the surface of a cube homeomorphic to a sphere $S^3$ by retraction, but I'm confused that whether the surface of a cube can be diffeomorphic to $S^3$.










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  • Do you mean the surface of the unit hypercube in $Bbb R^4$, with the differential structure inherited from $Bbb R^4$? I don't really know what happens at the edges and vertices there, but it looks messy.
    – Arthur
    Dec 2 '18 at 14:51












  • Can you define what you mean by "3-dimensional cube" and by "3-dimensional sphere"? I would think that the former is the three-fold product of the unit interval, and the latter is the unit sphere in $Bbb R^4$. But those two spaces are not homeomorphic (e.g. their homology distinguishes them).
    – Danu
    Dec 2 '18 at 15:34










  • I think you mean a $3$ dimensional ball, which is a solid object like the earth. We use sphere to represent the surface of a ball.
    – Ross Millikan
    Dec 2 '18 at 16:07












  • In addition, in order to ask if two things are diffeomorphic, both things must be smooth manifolds. So, you must first choose some kind of atlas on the cube.
    – Jason DeVito
    Dec 4 '18 at 14:24










  • So the question is, if there exist a kind of atlas on cube, such that the cube with the atlas can be a smooth manifolds.
    – Lau
    Dec 4 '18 at 14:26
















0














We all know the surface of a cube homeomorphic to a sphere $S^3$ by retraction, but I'm confused that whether the surface of a cube can be diffeomorphic to $S^3$.










share|cite|improve this question
























  • Do you mean the surface of the unit hypercube in $Bbb R^4$, with the differential structure inherited from $Bbb R^4$? I don't really know what happens at the edges and vertices there, but it looks messy.
    – Arthur
    Dec 2 '18 at 14:51












  • Can you define what you mean by "3-dimensional cube" and by "3-dimensional sphere"? I would think that the former is the three-fold product of the unit interval, and the latter is the unit sphere in $Bbb R^4$. But those two spaces are not homeomorphic (e.g. their homology distinguishes them).
    – Danu
    Dec 2 '18 at 15:34










  • I think you mean a $3$ dimensional ball, which is a solid object like the earth. We use sphere to represent the surface of a ball.
    – Ross Millikan
    Dec 2 '18 at 16:07












  • In addition, in order to ask if two things are diffeomorphic, both things must be smooth manifolds. So, you must first choose some kind of atlas on the cube.
    – Jason DeVito
    Dec 4 '18 at 14:24










  • So the question is, if there exist a kind of atlas on cube, such that the cube with the atlas can be a smooth manifolds.
    – Lau
    Dec 4 '18 at 14:26














0












0








0







We all know the surface of a cube homeomorphic to a sphere $S^3$ by retraction, but I'm confused that whether the surface of a cube can be diffeomorphic to $S^3$.










share|cite|improve this question















We all know the surface of a cube homeomorphic to a sphere $S^3$ by retraction, but I'm confused that whether the surface of a cube can be diffeomorphic to $S^3$.







smooth-manifolds






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Dec 4 '18 at 14:27

























asked Dec 2 '18 at 14:35









Lau

541315




541315












  • Do you mean the surface of the unit hypercube in $Bbb R^4$, with the differential structure inherited from $Bbb R^4$? I don't really know what happens at the edges and vertices there, but it looks messy.
    – Arthur
    Dec 2 '18 at 14:51












  • Can you define what you mean by "3-dimensional cube" and by "3-dimensional sphere"? I would think that the former is the three-fold product of the unit interval, and the latter is the unit sphere in $Bbb R^4$. But those two spaces are not homeomorphic (e.g. their homology distinguishes them).
    – Danu
    Dec 2 '18 at 15:34










  • I think you mean a $3$ dimensional ball, which is a solid object like the earth. We use sphere to represent the surface of a ball.
    – Ross Millikan
    Dec 2 '18 at 16:07












  • In addition, in order to ask if two things are diffeomorphic, both things must be smooth manifolds. So, you must first choose some kind of atlas on the cube.
    – Jason DeVito
    Dec 4 '18 at 14:24










  • So the question is, if there exist a kind of atlas on cube, such that the cube with the atlas can be a smooth manifolds.
    – Lau
    Dec 4 '18 at 14:26


















  • Do you mean the surface of the unit hypercube in $Bbb R^4$, with the differential structure inherited from $Bbb R^4$? I don't really know what happens at the edges and vertices there, but it looks messy.
    – Arthur
    Dec 2 '18 at 14:51












  • Can you define what you mean by "3-dimensional cube" and by "3-dimensional sphere"? I would think that the former is the three-fold product of the unit interval, and the latter is the unit sphere in $Bbb R^4$. But those two spaces are not homeomorphic (e.g. their homology distinguishes them).
    – Danu
    Dec 2 '18 at 15:34










  • I think you mean a $3$ dimensional ball, which is a solid object like the earth. We use sphere to represent the surface of a ball.
    – Ross Millikan
    Dec 2 '18 at 16:07












  • In addition, in order to ask if two things are diffeomorphic, both things must be smooth manifolds. So, you must first choose some kind of atlas on the cube.
    – Jason DeVito
    Dec 4 '18 at 14:24










  • So the question is, if there exist a kind of atlas on cube, such that the cube with the atlas can be a smooth manifolds.
    – Lau
    Dec 4 '18 at 14:26
















Do you mean the surface of the unit hypercube in $Bbb R^4$, with the differential structure inherited from $Bbb R^4$? I don't really know what happens at the edges and vertices there, but it looks messy.
– Arthur
Dec 2 '18 at 14:51






Do you mean the surface of the unit hypercube in $Bbb R^4$, with the differential structure inherited from $Bbb R^4$? I don't really know what happens at the edges and vertices there, but it looks messy.
– Arthur
Dec 2 '18 at 14:51














Can you define what you mean by "3-dimensional cube" and by "3-dimensional sphere"? I would think that the former is the three-fold product of the unit interval, and the latter is the unit sphere in $Bbb R^4$. But those two spaces are not homeomorphic (e.g. their homology distinguishes them).
– Danu
Dec 2 '18 at 15:34




Can you define what you mean by "3-dimensional cube" and by "3-dimensional sphere"? I would think that the former is the three-fold product of the unit interval, and the latter is the unit sphere in $Bbb R^4$. But those two spaces are not homeomorphic (e.g. their homology distinguishes them).
– Danu
Dec 2 '18 at 15:34












I think you mean a $3$ dimensional ball, which is a solid object like the earth. We use sphere to represent the surface of a ball.
– Ross Millikan
Dec 2 '18 at 16:07






I think you mean a $3$ dimensional ball, which is a solid object like the earth. We use sphere to represent the surface of a ball.
– Ross Millikan
Dec 2 '18 at 16:07














In addition, in order to ask if two things are diffeomorphic, both things must be smooth manifolds. So, you must first choose some kind of atlas on the cube.
– Jason DeVito
Dec 4 '18 at 14:24




In addition, in order to ask if two things are diffeomorphic, both things must be smooth manifolds. So, you must first choose some kind of atlas on the cube.
– Jason DeVito
Dec 4 '18 at 14:24












So the question is, if there exist a kind of atlas on cube, such that the cube with the atlas can be a smooth manifolds.
– Lau
Dec 4 '18 at 14:26




So the question is, if there exist a kind of atlas on cube, such that the cube with the atlas can be a smooth manifolds.
– Lau
Dec 4 '18 at 14:26










1 Answer
1






active

oldest

votes


















2














Let me answer this in lower dimensions:



First, consider the bi-unit square $S$ (the boundary of $[-1, 1] times [-1, 1]$ in $Bbb R^2$) and the unit circle $C$ in $Bbb R^2$.



There's a nice map ("radial projection") from the first to the second defined as follows:



$$
f(x, y) = begin{cases}
(frac{x}{sqrt{y^2+1}}, frac{y}{sqrt{y^2 + 1}})& x = pm 1 \
(frac{x}{sqrt{x^2+1}}, frac{y}{sqrt{x^2 + 1}})& y = pm 1 \
end{cases}
$$

which cane be written, more generally, as $f(x, y) = frac{1}{sqrt{x^2 + y^2}} (x, y)$, but I wanted to emphasize that it's defined on the square, and that at the four corners, the expressions involved shift from a dependence on $x$ to a dependence on $y$ or vice versa.



The inverse of $f$ is
$$
g(x, y) = begin{cases}
(frac{x}{|x|}, frac{y}{|x|}) & |x| ge |y|\
(frac{x}{|y|}, frac{y}{|y|}) & |x| le |y|
end{cases}
$$



Now one smoothness structure on $S$ can be described as follows: we say that a function
$$
u: S to Bbb R
$$
is smooth if and only iff
$$
u circ g : C to Bbb R
$$

is smooth.



With this definition, the square $S$ is a smooth manifold, and it's diffeomorphic to the circle $C$, with $g$ being the diffeomorphism.



There's a small caveat here, though: the inclusion map
$$
i : S to Bbb R^2: (x, y) mapsto (x, y)
$$

that embeds the square in the plane is not a smooth embedding.



What about higher dimensions? Well, an exact analog to $f$ can be written using more variables (and the "general" formula that I gave after the "cases" formula); the analog of $g$ involves 3 cases for a cube in 3-space (look at $|x|, |y|, |z|$: each case handles the situations where one of these is the largest).



When we get to 4-space, it's the same deal, but with another variable, and the formula for $g$ has four cases.



But everything else I said (including the fact that with the resulting smoothness structure, the standard embedding of the square/cube/hypercube is not a smooth embedding) remains true.






share|cite|improve this answer























  • what's $s$ in $scirc g$?
    – Lau
    Dec 5 '18 at 3:16










  • Fixed. It's "u" -- just a typo. Sorry about that.
    – John Hughes
    Dec 5 '18 at 12:44










  • I think I have got your point, thanks a lot.
    – Lau
    Dec 5 '18 at 12:47











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1 Answer
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1 Answer
1






active

oldest

votes









active

oldest

votes






active

oldest

votes









2














Let me answer this in lower dimensions:



First, consider the bi-unit square $S$ (the boundary of $[-1, 1] times [-1, 1]$ in $Bbb R^2$) and the unit circle $C$ in $Bbb R^2$.



There's a nice map ("radial projection") from the first to the second defined as follows:



$$
f(x, y) = begin{cases}
(frac{x}{sqrt{y^2+1}}, frac{y}{sqrt{y^2 + 1}})& x = pm 1 \
(frac{x}{sqrt{x^2+1}}, frac{y}{sqrt{x^2 + 1}})& y = pm 1 \
end{cases}
$$

which cane be written, more generally, as $f(x, y) = frac{1}{sqrt{x^2 + y^2}} (x, y)$, but I wanted to emphasize that it's defined on the square, and that at the four corners, the expressions involved shift from a dependence on $x$ to a dependence on $y$ or vice versa.



The inverse of $f$ is
$$
g(x, y) = begin{cases}
(frac{x}{|x|}, frac{y}{|x|}) & |x| ge |y|\
(frac{x}{|y|}, frac{y}{|y|}) & |x| le |y|
end{cases}
$$



Now one smoothness structure on $S$ can be described as follows: we say that a function
$$
u: S to Bbb R
$$
is smooth if and only iff
$$
u circ g : C to Bbb R
$$

is smooth.



With this definition, the square $S$ is a smooth manifold, and it's diffeomorphic to the circle $C$, with $g$ being the diffeomorphism.



There's a small caveat here, though: the inclusion map
$$
i : S to Bbb R^2: (x, y) mapsto (x, y)
$$

that embeds the square in the plane is not a smooth embedding.



What about higher dimensions? Well, an exact analog to $f$ can be written using more variables (and the "general" formula that I gave after the "cases" formula); the analog of $g$ involves 3 cases for a cube in 3-space (look at $|x|, |y|, |z|$: each case handles the situations where one of these is the largest).



When we get to 4-space, it's the same deal, but with another variable, and the formula for $g$ has four cases.



But everything else I said (including the fact that with the resulting smoothness structure, the standard embedding of the square/cube/hypercube is not a smooth embedding) remains true.






share|cite|improve this answer























  • what's $s$ in $scirc g$?
    – Lau
    Dec 5 '18 at 3:16










  • Fixed. It's "u" -- just a typo. Sorry about that.
    – John Hughes
    Dec 5 '18 at 12:44










  • I think I have got your point, thanks a lot.
    – Lau
    Dec 5 '18 at 12:47
















2














Let me answer this in lower dimensions:



First, consider the bi-unit square $S$ (the boundary of $[-1, 1] times [-1, 1]$ in $Bbb R^2$) and the unit circle $C$ in $Bbb R^2$.



There's a nice map ("radial projection") from the first to the second defined as follows:



$$
f(x, y) = begin{cases}
(frac{x}{sqrt{y^2+1}}, frac{y}{sqrt{y^2 + 1}})& x = pm 1 \
(frac{x}{sqrt{x^2+1}}, frac{y}{sqrt{x^2 + 1}})& y = pm 1 \
end{cases}
$$

which cane be written, more generally, as $f(x, y) = frac{1}{sqrt{x^2 + y^2}} (x, y)$, but I wanted to emphasize that it's defined on the square, and that at the four corners, the expressions involved shift from a dependence on $x$ to a dependence on $y$ or vice versa.



The inverse of $f$ is
$$
g(x, y) = begin{cases}
(frac{x}{|x|}, frac{y}{|x|}) & |x| ge |y|\
(frac{x}{|y|}, frac{y}{|y|}) & |x| le |y|
end{cases}
$$



Now one smoothness structure on $S$ can be described as follows: we say that a function
$$
u: S to Bbb R
$$
is smooth if and only iff
$$
u circ g : C to Bbb R
$$

is smooth.



With this definition, the square $S$ is a smooth manifold, and it's diffeomorphic to the circle $C$, with $g$ being the diffeomorphism.



There's a small caveat here, though: the inclusion map
$$
i : S to Bbb R^2: (x, y) mapsto (x, y)
$$

that embeds the square in the plane is not a smooth embedding.



What about higher dimensions? Well, an exact analog to $f$ can be written using more variables (and the "general" formula that I gave after the "cases" formula); the analog of $g$ involves 3 cases for a cube in 3-space (look at $|x|, |y|, |z|$: each case handles the situations where one of these is the largest).



When we get to 4-space, it's the same deal, but with another variable, and the formula for $g$ has four cases.



But everything else I said (including the fact that with the resulting smoothness structure, the standard embedding of the square/cube/hypercube is not a smooth embedding) remains true.






share|cite|improve this answer























  • what's $s$ in $scirc g$?
    – Lau
    Dec 5 '18 at 3:16










  • Fixed. It's "u" -- just a typo. Sorry about that.
    – John Hughes
    Dec 5 '18 at 12:44










  • I think I have got your point, thanks a lot.
    – Lau
    Dec 5 '18 at 12:47














2












2








2






Let me answer this in lower dimensions:



First, consider the bi-unit square $S$ (the boundary of $[-1, 1] times [-1, 1]$ in $Bbb R^2$) and the unit circle $C$ in $Bbb R^2$.



There's a nice map ("radial projection") from the first to the second defined as follows:



$$
f(x, y) = begin{cases}
(frac{x}{sqrt{y^2+1}}, frac{y}{sqrt{y^2 + 1}})& x = pm 1 \
(frac{x}{sqrt{x^2+1}}, frac{y}{sqrt{x^2 + 1}})& y = pm 1 \
end{cases}
$$

which cane be written, more generally, as $f(x, y) = frac{1}{sqrt{x^2 + y^2}} (x, y)$, but I wanted to emphasize that it's defined on the square, and that at the four corners, the expressions involved shift from a dependence on $x$ to a dependence on $y$ or vice versa.



The inverse of $f$ is
$$
g(x, y) = begin{cases}
(frac{x}{|x|}, frac{y}{|x|}) & |x| ge |y|\
(frac{x}{|y|}, frac{y}{|y|}) & |x| le |y|
end{cases}
$$



Now one smoothness structure on $S$ can be described as follows: we say that a function
$$
u: S to Bbb R
$$
is smooth if and only iff
$$
u circ g : C to Bbb R
$$

is smooth.



With this definition, the square $S$ is a smooth manifold, and it's diffeomorphic to the circle $C$, with $g$ being the diffeomorphism.



There's a small caveat here, though: the inclusion map
$$
i : S to Bbb R^2: (x, y) mapsto (x, y)
$$

that embeds the square in the plane is not a smooth embedding.



What about higher dimensions? Well, an exact analog to $f$ can be written using more variables (and the "general" formula that I gave after the "cases" formula); the analog of $g$ involves 3 cases for a cube in 3-space (look at $|x|, |y|, |z|$: each case handles the situations where one of these is the largest).



When we get to 4-space, it's the same deal, but with another variable, and the formula for $g$ has four cases.



But everything else I said (including the fact that with the resulting smoothness structure, the standard embedding of the square/cube/hypercube is not a smooth embedding) remains true.






share|cite|improve this answer














Let me answer this in lower dimensions:



First, consider the bi-unit square $S$ (the boundary of $[-1, 1] times [-1, 1]$ in $Bbb R^2$) and the unit circle $C$ in $Bbb R^2$.



There's a nice map ("radial projection") from the first to the second defined as follows:



$$
f(x, y) = begin{cases}
(frac{x}{sqrt{y^2+1}}, frac{y}{sqrt{y^2 + 1}})& x = pm 1 \
(frac{x}{sqrt{x^2+1}}, frac{y}{sqrt{x^2 + 1}})& y = pm 1 \
end{cases}
$$

which cane be written, more generally, as $f(x, y) = frac{1}{sqrt{x^2 + y^2}} (x, y)$, but I wanted to emphasize that it's defined on the square, and that at the four corners, the expressions involved shift from a dependence on $x$ to a dependence on $y$ or vice versa.



The inverse of $f$ is
$$
g(x, y) = begin{cases}
(frac{x}{|x|}, frac{y}{|x|}) & |x| ge |y|\
(frac{x}{|y|}, frac{y}{|y|}) & |x| le |y|
end{cases}
$$



Now one smoothness structure on $S$ can be described as follows: we say that a function
$$
u: S to Bbb R
$$
is smooth if and only iff
$$
u circ g : C to Bbb R
$$

is smooth.



With this definition, the square $S$ is a smooth manifold, and it's diffeomorphic to the circle $C$, with $g$ being the diffeomorphism.



There's a small caveat here, though: the inclusion map
$$
i : S to Bbb R^2: (x, y) mapsto (x, y)
$$

that embeds the square in the plane is not a smooth embedding.



What about higher dimensions? Well, an exact analog to $f$ can be written using more variables (and the "general" formula that I gave after the "cases" formula); the analog of $g$ involves 3 cases for a cube in 3-space (look at $|x|, |y|, |z|$: each case handles the situations where one of these is the largest).



When we get to 4-space, it's the same deal, but with another variable, and the formula for $g$ has four cases.



But everything else I said (including the fact that with the resulting smoothness structure, the standard embedding of the square/cube/hypercube is not a smooth embedding) remains true.







share|cite|improve this answer














share|cite|improve this answer



share|cite|improve this answer








edited Dec 5 '18 at 12:44

























answered Dec 4 '18 at 14:43









John Hughes

62.4k24090




62.4k24090












  • what's $s$ in $scirc g$?
    – Lau
    Dec 5 '18 at 3:16










  • Fixed. It's "u" -- just a typo. Sorry about that.
    – John Hughes
    Dec 5 '18 at 12:44










  • I think I have got your point, thanks a lot.
    – Lau
    Dec 5 '18 at 12:47


















  • what's $s$ in $scirc g$?
    – Lau
    Dec 5 '18 at 3:16










  • Fixed. It's "u" -- just a typo. Sorry about that.
    – John Hughes
    Dec 5 '18 at 12:44










  • I think I have got your point, thanks a lot.
    – Lau
    Dec 5 '18 at 12:47
















what's $s$ in $scirc g$?
– Lau
Dec 5 '18 at 3:16




what's $s$ in $scirc g$?
– Lau
Dec 5 '18 at 3:16












Fixed. It's "u" -- just a typo. Sorry about that.
– John Hughes
Dec 5 '18 at 12:44




Fixed. It's "u" -- just a typo. Sorry about that.
– John Hughes
Dec 5 '18 at 12:44












I think I have got your point, thanks a lot.
– Lau
Dec 5 '18 at 12:47




I think I have got your point, thanks a lot.
– Lau
Dec 5 '18 at 12:47


















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