Give an example or prove that it is impossible:

In summary, a sequence of Lipschitz functions converging uniformly to a non-Lipschitz function does not necessarily result in the final function being Lipschitz. An example of this is a polygonal approximation to the upper half of a circle, which is not Lipschitz even though the approximating functions are.
  • #1
davitykale
38
0

Homework Statement


A sequence of Lipschitz functions f_n: [0,1] --> R which converges uniformly to a non-Lipschitz function


Homework Equations


a function f: A --> R is Lipschitz if there exists a constant M \in R such that |f(x)-f(y)|<=M|x-y|


The Attempt at a Solution


I don't think it's possible but I'm not sure how to prove that this is the case
 
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  • #2
Think about a polygonal approximation to the upper half of a circle.
 
  • #3
Sorry, I'm really confused :/
 
  • #4
LCKurtz said:
Think about a polygonal approximation to the upper half of a circle.

davitykale said:
Sorry, I'm really confused :/

Look at the top half of x2+y2=1. Mark the points on that semicircle that correspond to x = -1,-1/2,0,1/2, and 1. Join these points with straight line segments. That would give what is called a polygonal approximation to the curve with 4 segments. You might call that function f4(x). Think about fn(x).
 
  • #5
How is f not Lipschitz if f_n(x) is? Doesn't f_n --> f where f is the semicircle?
 
  • #6
davitykale said:
How is f not Lipschitz if f_n(x) is? Doesn't f_n --> f where f is the semicircle?

There is no reason for f to be Lipschitz even if the fn are. Doesn't the (beautiful) example of LCKurtz show this?
 
  • #7
Maybe I'm just confused...f is the semicircle, correct?
 
  • #8
davitykale said:
Maybe I'm just confused...f is the semicircle, correct?

Yes, but any non-Lipschitz function will work...
 
  • #9
Thread locked.
 
Last edited:

FAQ: Give an example or prove that it is impossible:

Can you give an example of something that is impossible to prove?

Yes, one example is the statement "There is no highest prime number." This statement is impossible to prove because even if we search for the highest prime number, there could always be a larger prime number that we haven't found yet.

Is there a way to prove that something is impossible?

No, there is no way to definitively prove that something is impossible. We can only provide evidence or logical reasoning to support the idea that something is impossible, but we cannot prove it with absolute certainty.

How do you know when something is impossible?

We can determine that something is impossible by examining all available evidence and using logical reasoning to conclude that it cannot be achieved or proven. However, there may always be a chance that new evidence or technology could change our understanding of what is possible.

Can you prove that something is impossible using mathematics?

Yes, mathematics can be used to prove that something is impossible in certain scenarios. For example, the proof of the impossibility of trisecting an angle using only a compass and straightedge relies on mathematical principles.

Is it possible for something to be impossible in theory, but possible in practice?

Yes, it is possible for something to be theoretically impossible, but still achievable in practice. For example, it was once thought to be impossible for humans to run a mile in under four minutes, but it has since been achieved by many athletes. So while it was impossible in theory, it was possible in practice with the right training and conditions.

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