Understanding Extremum and Derivatives at Boundary Points

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    Boundary Extremum
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Discussion Overview

The discussion revolves around the behavior of derivatives at boundary points of functions, particularly in the context of finding absolute extrema under restricted boundary conditions. Participants explore the implications of continuity and dimensionality on the existence of extrema at boundaries, as well as the nature of topological boundaries.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions why the derivative does not indicate an extremum at boundary points, suggesting that it only identifies extrema at interior points.
  • Another participant asserts that for continuous functions, the derivative at the boundary is irrelevant, as the function value at the boundary will be an extremum compared to nearby values, but this is limited to one-dimensional functions.
  • A later reply clarifies that in higher dimensions, if the boundary has a dimension greater than zero, the derivative must be zero along the boundary for an extremum to exist there.
  • One participant challenges the notion that topological boundaries must be zero-dimensional, providing an example of a circular boundary and emphasizing that boundary points must be within the set to yield local extrema.
  • Another participant acknowledges the complexity of the discussion, suggesting it may be too advanced for the current thread context.

Areas of Agreement / Disagreement

Participants express differing views on the nature of boundaries and the conditions under which extrema can occur at boundary points. There is no consensus on the dimensionality of topological boundaries or the implications for extrema.

Contextual Notes

Participants reference continuity, dimensionality, and the necessity of boundary points being within the set for local extrema, but these concepts remain unresolved and are subject to interpretation.

Leo Authersh
It's understandable that finding absolute extremum is impossible for a function with restricted boundary conditions. But why does the derivative of similar functions is not zero when the extremum is on the end points?

To be precisely short with my question, why does the derivative gives only the extremum at the interior points within the boundary and not at the points on the boundary?
 
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In short, assuming that the function is continuous, because it does not matter what the derivative is on the boundary. Regardless of the derivative the value of the function at the boundary will be larger than or smaller all other function values in a small region close to the boundary.

Note that this holds only for one-dimensional functions. If you deal with functions in several dimensions and the boundary has a dimension greater than zero, then the derivative along the boundary must be zero for there to be an extremum on the boundary.
 
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Orodruin said:
In short, assuming that the function is continuous, because it does not matter what the derivative is on the boundary. Regardless of the derivative the value of the function at the boundary will be larger than or smaller all other function values in a small region close to the boundary.

Note that this holds only for one-dimensional functions. If you deal with functions in several dimensions and the boundary has a dimension greater than zero, then the derivative along the boundary must be zero for there to be an extremum on the boundary.
This is the topological boundary, right? Aren't Topological boundaries necessarily zero-dimensional? Or do you mean Manifold boundaries?
 
Why would the topological boundary need to be zero dimensional? Consider the topological boundary of the set ##x^2+y^2\leq 1##, which is a circle. Anyway, an important issue is that the boundary point must also be in the set in order to provide a local extremum.

However, I believe this discussion may be too advanced for a B thread.
 
Orodruin said:
Why would the topological boundary need to be zero dimensional? Consider the topological boundary of the set ##x^2+y^2\leq 1##, which is a circle. Anyway, an important issue is that the boundary point must also be in the set in order to provide a local extremum.

However, I believe this discussion may be too advanced for a B thread.
Sorry, you're right, I think it was some measure of " meagreness" in the ambient space. But you're right, let's drop it and save it for some other post.
 

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