Lagrange Multiplier where constraint is a rectangle

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Discussion Overview

The discussion revolves around the application of Lagrange Multipliers to find the extrema of the function f(x,y) = x² + 4y² - 2x²y + 4 over a rectangular region defined by the constraints -1 ≤ x ≤ 1 and -1 ≤ y ≤ 1. The scope includes theoretical approaches and mathematical reasoning related to optimization within constrained regions.

Discussion Character

  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant suggests using Lagrange Multipliers to find extrema on the boundary of the rectangle after studying critical points of the gradient inside the region.
  • Another participant proposes that Lagrange multipliers are primarily useful for continuous constraints and recommends first finding extrema without considering the rectangle, then checking if those points lie within the rectangle before applying Lagrange multipliers to the boundaries if necessary.
  • A different viewpoint argues that using Lagrange multipliers for the boundaries may be excessive, suggesting instead to directly substitute the boundary constraints into the function and treat it as a function of the remaining variable.

Areas of Agreement / Disagreement

Participants express differing opinions on the necessity and appropriateness of using Lagrange multipliers for this problem, indicating that there is no consensus on the best approach to take.

Contextual Notes

Participants do not fully resolve the implications of using Lagrange multipliers versus other methods, and there is ambiguity regarding the handling of constraints that are not strictly continuous.

SamitC
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Hello,
How can I use Lagrange Multipliers to get the Extrema of a curve f(x,y) = x2+4y2-2x2y+4 over a rectangular region -1<=x<=1 and -1<=y<=1 ?
Thanks
 
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yes, before it is simpler to study the critical point of the gradient and look for max or min inside the rectangle, you can apply the Lagrange method for the boundary of the rectangle ...
 
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Just to add to what Ssnow said, I think that Lagrange multipliers are only directly useful for continuous constraints. If the constraint is just that (x,y) must be inside a rectangle, then I would think that you would do the following:

  1. First, find the extrema forgetting about the rectangle.
  2. Then if the extrema found in step 1 are all inside the rectangle, then you're done.
  3. If not, then use the method of Lagrange multipliers to find the extrema for each of the four sides of the rectangle.
 
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I actually think it is overkill to use Lagrange multipliers for the boundaries. Since the boundaries are so simple, just insert the boundary constraint and treat the resulting function as a function of the remaining variable.
 
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Thanks a lot. Its clear now.
 

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