Identifying local maximum, local minimum and saddle point.

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Discussion Overview

The discussion revolves around identifying local maxima, minima, and saddle points in a mathematical context, specifically analyzing a contour plot. Participants are attempting to determine the correctness of their classifications based on the visual representation of the function.

Discussion Character

  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant identifies Point P as a local maximum, Point Q as a local minimum, Point R as none, and Point S as a saddle point, but expresses uncertainty about one incorrect classification.
  • Another participant raises concerns about the labeling of contours around Points Q and R, suggesting that the signs associated with these points may be incorrect and that this affects the classification of local extrema.
  • A later reply reiterates the confusion regarding the contour around Q, questioning the placement of the zero contour and suggesting that important information may be missing from the plot.
  • Further discussion highlights the necessity of a zero contour between the -1 and +1 contours for a continuous function, questioning the implications of the existing contours and their relationships.

Areas of Agreement / Disagreement

Participants express disagreement regarding the correct classification of Points Q and R, with multiple competing views on the implications of the contour plot and the potential for omitted information.

Contextual Notes

There are unresolved issues regarding the assumptions about the continuity of the function and the accuracy of the contour labels, which may affect the conclusions drawn by participants.

BondKing
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Indicate whether you think it is a local maximum, local minimum, saddle point, or none of these?

615d6b88-0905-30da-8b9f-5d95379e82e4___38df22d3-72ff-3595-8d9b-53259de06ff3_zpsc9b9bd2b.png

My solution:

Point P = Local Max
Point Q = Local Min
Point R = None
Point S = Saddle

I got a 75% for first attempt, so one answer is not correct and I am not sure which one isn't.
 
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The circle around Q is confusing. If it is really +1, it should have a zero ring around it. If it is -1 (what I would expect), where is the minus sign?
Same problem with the circle to the right of R, just with reversed signs.

Either R or Q give the issue, which one depends on those sign problems.
 
mfb said:
The circle around Q is confusing. If it is really +1, it should have a zero ring around it. If it is -1 (what I would expect), where is the minus sign?
Same problem with the circle to the right of R, just with reversed signs.

Either R or Q give the issue, which one depends on those sign problems.

Actually the problem with Q is bigger than that. The 0 contour is marked in the picture: it is the pair of diagonal lines going through S. Given the 1 contour around Q, where should the 0 contour go? There must be one between -1 and 1 (unless the function is discontinuous on a ring around Q, in which case the problem is evil). But if the 0 contour is a ring around Q, then what to make of the 0 contour that is already drawn going through S? Either these contours must join up (possibly at R?), or there must be an additional local min/max somewhere between Q and the origin. Either way, it seems that important information has been omitted from the plot.
 
Ben Niehoff said:
The 0 contour is marked in the picture: it is the pair of diagonal lines going through S.
This cannot be the whole 0 contour for a continuous function. There is a path from a -1 to a +1 contour which has to have a 0 somewhere.
Ben Niehoff said:
But if the 0 contour is a ring around Q, then what to make of the 0 contour that is already drawn going through S?
Where is the problem? (x^2-y^2)^2*((x-1)^2+y^2-0.1) has this type of contours.
Ben Niehoff said:
or there must be an additional local min/max somewhere between Q and the origin.
It does not have to, but even if it has, where is the problem?
Ben Niehoff said:
Either way, it seems that important information has been omitted from the plot.
Certainly, as there is no 0 contour or the contour labels are wrong.
 

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