- #1
iorfus
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Homework Statement
I have to prove some things on the Weber-Ferma problem. Here is the assignment :
We want to find a point $$x$$ in the plane whose sum of weighted
distances from a given set of fixed points $$y_1, ...,y_m$$ is minimized.
1-Show that there exist a global mimimum to the problem.
2-Is the minimum always unique?
3-Considering that there are actually ropes connected to weights for each of the fixed point, prove that the minimum is the point where the resultant force is zero.
My problem is that I want to prove the las point by observing that the gradient is just the resulting force on the minimum. Therefore, being the gradient zero for a minimum, the point is proven.
However, I have two questions which I am not able to answer:
1-If the points are not collinear, can the minimum be one of them?
2-If the mimimum is one of them, how can I prove that the resulting force is null?
Forum-related question : $x$ does not work, is it normal?
Thanks
Homework Equations
The function to minimize is
$$f(x)= \sum_{i=1}^m w_i|| x-y_i || $$
with no constraints.
The Attempt at a Solution
1-Done, the function is continuous and I can define it on a compact ball of arbitrary radius, so for Weierstrass's Theorem it has a minimum.
2-The minimum is unique if the points are not collinear. Indeed, the function f(x), which is convex, can be shown to be strictly convex if the points are not collinear (if someone is interested, I will report my proof).
3-It should be easy, the gradient is null in the minimum and the gradient is just the resultant force.
My problem is : what do I do if the minimum coincides with one of the fixed point? I cannot differentiate, the gradient is not defined.