Understanding Closed Curves & Remark on Unique Extension

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Discussion Overview

The discussion revolves around the concept of extending closed curves, particularly focusing on the conditions under which a curve can be uniquely extended to a periodic smooth curve. Participants explore the implications of having equal values and derivatives at specific points and the nature of smoothness in this context.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions the meaning of the statement regarding the unique extension of a curve and suggests that equal derivatives at endpoints prevent sharp edges, implying a need for smoothness.
  • Another participant asserts that the condition ensures the extension is smooth, using the sine function as an example of a periodic function that meets these criteria.
  • A later reply clarifies that while the theorem establishes smoothness for periodic extensions, it does not imply that there is a unique way to extend all smooth curves, nor does it exclude the possibility of extending non-smooth functions to be periodic.
  • An example is provided of a function that is smooth at a point but can be extended in multiple ways, challenging the notion of uniqueness in extensions.

Areas of Agreement / Disagreement

Participants express differing views on the uniqueness of extensions and the implications of smoothness, indicating that the discussion remains unresolved regarding the broader applicability of the theorem and the nature of extensions.

Contextual Notes

Some participants highlight that the discussion may depend on specific definitions of smoothness and periodicity, and there are unresolved aspects regarding the conditions under which extensions can be made.

Buri
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I have a question about the Remark on the page posted. When it says "If γ and all its derivatives take the same value at a and b, there is a unique way to extend γ to a (b − a)-periodic (smooth) curve γ : R → R^n" what does this exactly mean? I suppose that the condition that the derivatives have the same value at a and b is to guarantee that there won't be any sharp edges at a and b right? So it eliminates the possibilibilty of piece-wise smooth curves? And can someone please explain to me what they when when they say "there is a unique way to extend γ to a (b - a)-periodic (smooth) curve γ: R → R^n"?

Thanks for any help you guys can give me.
 

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there is always a unique way to extend it as a periodic function, the condition makes the extension smooth.

e.g. look att he graph of the sine function. apparently it has the same value and same (one sided) derivatives at 0 and at 2pi, so that why it extends as a continuous smooth periodic curve to the whole line.
 
mathwonk said:
there is always a unique way to extend it as a periodic function, the condition makes the extension smooth.

e.g. look att he graph of the sine function. apparently it has the same value and same (one sided) derivatives at 0 and at 2pi, so that why it extends as a continuous smooth periodic curve to the whole line.

Ahhh I see. Thanks a lot for your help! :smile:
 
>"If γ and all its derivatives take the same value at a and b, there is a unique way to extend γ to a (b − a)-periodic (smooth) curve γ : R → R^n"

Note that this theorem just helps establish that the periodic result is smooth.
It doesn't at all mean there's a unique way of extending a smooth curve, or that non-smooth functions can't be extended to be periodic!

Consider the function g(x)=exp(-1/x^2) truncated to be 0 for x < 0. At x = 0, the function g and all its derivatives are equal to zero. So you can always add on copies of this function to any other function without affecting the result's smoothness.
 

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