What's the Secret Behind the Geometry Paradox of Increasing Triangles?

In summary, the conversation discussed the concept of uniform convergence and its implications for the length of geometric progressions. It was argued that while uniform convergence does not guarantee convergence of lengths, it does converge to the desired shape. The conversation also touched on the topic of the rectangle-circle problem and its relationship to the pi = 4 paradox. The discussion concluded with a visualization of the argument using a right triangle.
  • #1
Johnny B.
5
0
Check it out: http://imgur.com/EpYQv
Where's the trick?
 
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  • #2
The 10 line is never quite flat. In the limit it consists of a lot of infinitesimal wiggles.
 
  • #3
mathman said:
The 10 line is never quite flat. In the limit it consists of a lot of infinitesimal wiggles.

That is certainly not my intuition of the topic. The limit IS flat, and the functions mentioned in the example WILL converge uniformly to the flat line.

The only thing is that even uniform convergence does not imply convergence of the lengths. Indeed, calculating the length involves taking the derivative. And a uniform convergent sequence might not have a converging sequence of derivatives. That is the thing that's going on here!
 
  • #4
Isn't this kind of the same as the rectangle-circle problem that was discussed a little while back?
 
  • #5
Johnny B. said:
Check it out: http://imgur.com/EpYQv
Where's the trick?
I think the only trick is the bare assertion that 6 = 10.

The limit of the geometric progression is indeed the line segment. However, there's no reason to believe that the length of the limit of the geometric progression is equal to the limit of the length of the geometric progression...
 
  • #6
olivermsun said:
Isn't this kind of the same as the rectangle-circle problem that was discussed a little while back?
This kind of post would be greatly improved by a link...
 
  • #9
The basic error is in the assumption that if every term of a sequence has a certain property, the limit must have that property. That is obviously not true. For example, every term in the sequence {1/n} has the property that it is positive but the limit is not.
 
  • #10
micromass said:
Indeed, calculating the length involves taking the derivative. And a uniform convergent sequence might not have a converging sequence of derivatives.

That definitely makes sense. Thanks to all of you for replying!
 
  • #11
micromass said:
That is certainly not my intuition of the topic. The limit IS flat, and the functions mentioned in the example WILL converge uniformly to the flat line.

The only thing is that even uniform convergence does not imply convergence of the lengths. Indeed, calculating the length involves taking the derivative. And a uniform convergent sequence might not have a converging sequence of derivatives. That is the thing that's going on here!

is this the same explanation for the pi = 4 paradox? The cutting corners method will converge uniformly to the circle, but there may not exist a converging sequence of derivatives?
 
  • #12
wisvuze said:
is this the same explanation for the pi = 4 paradox? The cutting corners method will converge uniformly to the circle, but there may not exist a converging sequence of derivatives?

Yes, it's the same thing actually!
 
  • #13
Cool, thanks :)
I believe you can prove that the cutting corners thing *does* converge uniformly to the circle; you can define on a quadrant-by-quadrant basis functions f_n to represent the nth cut-corner spiky thing, and C to be the original circle ( or partial circle on each quadrant). Then, you can come up with a sequence of numbers M_n, which represent the distances between C and "bigger circles" ( and also engulfing, being bigger than the spiky thing ). You can make M_n converge, and so by the weierstrass M-test, the sequence {f_n} converges uniformly to the circle.

If the convergence of the spiky things is not even uniform, then there is no hope at all right? All that says, is that for some point on your spiky thing, after some n, the point will come arbitrarily close to the smooth curve. But, the ability to draw a picture like the one linked above, or the pi = 4 picture, with ALL points looking arbitrarily closer and closer to the smooth-curve, it seems like uniform convergence is guaranteed
 
  • #14
wisvuze said:
Cool, thanks :)
I believe you can prove that the cutting corners thing *does* converge uniformly to the circle; you can define on a quadrant-by-quadrant basis functions f_n to represent the nth cut-corner spiky thing, and C to be the original circle ( or partial circle on each quadrant). Then, you can come up with a sequence of numbers M_n, which represent the distances between C and "bigger circles" ( and also engulfing, being bigger than the spiky thing ). You can make M_n converge, and so by the weierstrass M-test, the sequence {f_n} converges uniformly to the circle.

If the convergence of the spiky things is not even uniform, then there is no hope at all right? All that says, is that for some point on your spiky thing, after some n, the point will come arbitrarily close to the smooth curve. But, the ability to draw a picture like the one linked above, or the pi = 4 picture, with ALL points looking arbitrarily closer and closer to the smooth-curve, it seems like uniform convergence is guaranteed

I think you're right here. I don't so any reason why there shouldn't be uniform convergence...
 
  • #15
well, I was discussing this with a friend of mine.. he was trying to tell me that the picture fails to work because while the points of the spiky thing converge pointwise ( every point does), the convergence is not uniform, and that is the problem. I didn't really understand what he was trying to get at; but he was arguing so fast, not giving me time to think, so I had to let it go aha

thanks :)
 
  • #16
IMHO it's easier to visualize with the sawtooth than with the pi=4 example.

You can distill the argument to this: suppose you have one right triangle with sides 3, 4, 5 (or whatever). Now start shrinking the triangle. Obviously the hypotenuse is going to get "closer and closer" to the base, but does the ratio of the sides ever change? Basic geometry tells you no -- similar triangles are similar triangles, and the hypotenuse is always longer by the exact same ratio no matter how much you shrink the triangle. You can add or multiply the respective sides of as many little triangles as you want, but that won't change the ratios either.
 
  • #17
Mathman in post 2 is right.As the number of triangles increases the length of each hypotenuse decreases but this is compensated for exactly by the fact that the number of hypotenuses increases.
For 1 triangle there are 2 hypotenuses each of length 5.00
For 2 triangles there are 4 hypotenuses each of length 2.50
For 4 triangles there are 8 hypotenuses each of length 1.25 and so on.

There is a simple mathematical relationship between number of hypotenuses and length of one hypotenuse and the total length always comes out to be 5*2 no matter how many triangles are used.
 

1. How can a shape have a finite area but an infinite perimeter?

This paradox is known as the Gabriel's Horn paradox, and it occurs when a shape has a finite volume but an infinite surface area. In this case, the shape is created by rotating the function y=1/x around the x-axis. While the shape appears to have a finite area, its perimeter continues to increase as the function approaches the y-axis, resulting in an infinite perimeter.

2. What is the explanation for Zeno's paradox in geometry?

Zeno's paradox is a philosophical paradox that raises the question of whether motion is possible if space and time can be infinitely divided. In geometry, this paradox can be explained by the concept of limits. Essentially, as the distance between two objects approaches zero, the time it takes for one object to reach the other also approaches zero, allowing for motion to occur.

3. How does the Banach-Tarski paradox work?

The Banach-Tarski paradox is a mathematical theorem that states a solid sphere can be divided into a finite number of pieces and reassembled into two identical copies of the original sphere. This is possible because the pieces can be rearranged and rotated in specific ways without altering their size or shape. However, this paradox violates the Law of Conservation of Volume, which states that the volume of a closed system cannot change.

4. Is it possible to solve the Möbius strip paradox?

The Möbius strip paradox, also known as the Möbius band paradox, is a topological paradox where a strip of paper with a half-twist is joined to form a loop with only one side and one edge. While it may seem impossible, this paradox can be solved by cutting the strip down the middle, resulting in two linked strips, each with two sides and two edges.

5. What is the solution to the barber paradox?

The barber paradox is a self-referential paradox that asks whether a barber can shave himself if and only if he does not shave himself. This creates a paradox as the barber cannot both shave and not shave himself at the same time. In geometry, this paradox can be resolved by considering it as a logical paradox rather than a mathematical one. The statement itself is self-contradictory and therefore has no solution.

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