It is known that there are some 3D closed shapes with infinite surface area, but only finite volume.
Is there a (continuously differentiable) curve f : R → R whose solid of revolution has a finite surface area but infinite volume?
Gabriel's horn is formed by taking the solid of revolution of the curve f : R + → R , f ( x ) = x + 1 1 . It has finite volume and infinite surface area since:
V A = ∫ 0 ∞ π ( x + 1 1 ) 2 d x = [ π x + 1 1 ] 0 ∞ = π , = ∫ 0 ∞ 2 π x 1 + ( ( x + 1 ) 2 − 1 ) 2 d x > 2 π ∫ 0 ∞ x 1 d x = [ 2 π ln ( x + 1 ) ] 0 ∞ = ∞ .
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A sphere is a surface of revolution, and there cannot be any surface of revolution that has an infinite volume while having a finite surface area.
I think you should offer a better example of a shape that has a finite surface area but encloses an infinite volume, without resorting to "legalisms" such as, "I define a sphere of finite radius that encloses everything outside of it". Or going to higher dimensions, such as proposing an infinite-dimensional sphere, etc.
If the shape has only a finite "diameter", i.e., the greatest distance between any two points on the surface, then one can enclose the shape with a sphere with a greater diameter, also finite. But this sphere has both a finite surface area and a finite volume, which contradicts the claim that the shape with a finite diameter has infinite volume. So, any 3D shape having these properties must at least 1) have an infinite diameter, and 2) NOT be a surface of revolution. What can it be? For this posted problem and answer to be complete, this should be described and explained.
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I suppose another "legalism" would be R 3 itself, which has infinite "volume" but no surface, so to speak, and thus no surface area. But if for an entity to be called a shape it must be contained within a distinct boundary, then could there be some kind of 3-D fractal construct that meets the requirements?
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We can conjure up other kinds of spaces that aren't necessarily Euclidean metric, i.e., curved spaces. As an example, suppose we were talking about a 2D "shape" instead of a 3D one, and we ask, "what 2D shape can have infinite area but finite perimeter?". We can create a bounded surface that has infinite area, and then mark a small circle of finite perimeter and area on it, and point out the shape that is NOT that small circle. Presto, we have a "shape" that has a finite perimeter and yet infinite area, and at the same time, no two points inside the shape are infinitely far apart.
If you need help with trying to imagine such a 2D bounded surface, think of airways in lungs, as a starting point. Lungs, in fact, are such that 1) it tries to maximize the surface area, while, 2) it tries to minimize the distance air has to travel. There's your fractal that you spoke of, but not quite exactly the same way you had envisioned.
We can generalize this idea from 2D to 3D, but it comes really hard to visualize such curved 3D spaces that have infinite volume but is of finite bound.
Is this just an exercise in legalism, or mathematics? In any case, I think this problem needs more precise wording instead of being so wildly open-ended as to be an exercise in philosophy.
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@Michael Mendrin – I like the lung analogy, (and I hadn't realized that lungs evolved to minimize the distance air had to travel), but I'm still finding it difficult to see how this generates a bounded surface of infinite rather than finite area. I revealed the solution without answering as the original wording was too vague. I think that the subject better lends itself to a discussion posting, such as Calvin posted many moons ago. I really need to take a deeper dive into topology someday, as it does seem fascinating on the surface, (sorry, never can resist puns :p).
No?
By using the isoperimetric inequality. In n-dimensional space R^n, the isoperimetric inequality lower bounds the surface area by it's volume. i.e...Volume constrains surface area?
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Going along with your thinking, in 3D we can slice up the shape into slabs, and then find cylinder sections that have the same volume but have less surface area, using the isoperimetric inequality. Then, aligning all such cylindrical slabs by their common axis, we end up with a surface of revolution which cannot have an infinite volume while having a finite surface area.
I think it would take some pretty extreme thinking to argue for the existence of a "shape" that would ahve these properties.
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Interesting and I agree. It would demonstrate why it is probably not possible, (unless one goes to absurd lengths to attempt to create them), for there to be shapes that have infinite volume and finite surface area. Not certain though- as I am not confident about my knowledge in this area.
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