Confirming Stationary Minima & Convexity of a Function

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Discussion Overview

The discussion revolves around the relationship between stationary points, local minima, and the convexity of functions. Participants explore whether having stationary minimum points implies convexity and whether a single stationary minimum point can be considered a global minimum. The scope includes theoretical aspects of calculus and function analysis.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • One participant questions if stationary points being minimum points qualifies the function as convex, suggesting that this is not necessarily true and providing a counterexample involving a piecewise function.
  • Another participant clarifies that having only one stationary minimum point does not guarantee it is a global minimum, citing the example of the function \(-x^4 + x^2\) which has a local minimum but a global minimum at infinity.
  • A further participant suggests that the shape of the graph could imply convexity, though this is not explicitly confirmed.
  • Another participant expresses uncertainty about the terminology, stating that they have not used "convex" to describe a 2-D Cartesian graph and describes the graph's concavity instead, mentioning the use of the second derivative test for points of inflection.

Areas of Agreement / Disagreement

Participants express differing views on the definitions and implications of convexity, stationary points, and local versus global minima. There is no consensus on these concepts, and the discussion remains unresolved.

Contextual Notes

Some assumptions about the definitions of stationary points and convexity may not be universally accepted. The discussion also highlights the potential for confusion regarding terminology in calculus.

ahamdiheme
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If the stationary points of a function are minimum points does that qualify the function to be a convex function?
Also, if the function has only one stationary minimum point does that mean that that point is its global minimum?
Can someone please confirm these for me
Thank you
 
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ahamdiheme said:
If the stationary points of a function are minimum points does that qualify the function to be a convex function?
Not necessarily. Consider the function f, whose graph is the union over all even integers n of the functions f_n(x) = (x + n)^2 each defined over the interval [n-1, n+1]. All of f's stationary points are minimum points, but f is not convex on [n, n + 2] for even n. Perhaps you mean critical points instead of stationary points?
 
ahamdiheme said:
Also, if the function has only one stationary minimum point does that mean that that point is its global minimum?
Thank you

If "stationary minimum" is the same as "local minimum", then not necessarily.

Look at the graph of -x^4 + x^2[/tex]. You can see that it has only one local minimum at x=0, but it's global minimum is at ±∞.
 
Considering the shape of the graph, that makes it convex right?
 
I may possibly not be very well versed in the subject, but I'm fairly sure that a graph is either "concave up", "concave down", or neither. I have never used the term convex to describe a 2-d Cartesian graph. The graph I supplied is concave for one interval and concave down at two intervals. You can solve at these points of inflection (where the graph changes from concave up to concave down) by using the second derivative test.

http://en.wikipedia.org/wiki/Second_derivative_test
 

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