I was working on a problem about geometric medians and I had an idea for a divide and conquer solution, but it would only work if a set of points, when split into two disjoint sets, and those sets respective geometric medians calculated and a line passed between, it holds that the geometric median of the overall set of points lies on that line. When graphing it and visualizing it, seems possible, and when I used existing algorithms and experimented, it seems to be very close but not quite on the same line. I am not sure if this is due to error in calculation of the geometric median or not.

I have some example Python code demonstrating how close the numbers are here: Geometric Median comparison

Any expertise in this matter would be appreciated.


1 Answer 1


If $A$ and $B$ are two set of points, the geometric median of $A \cup B$ does not necessarily lie on the line passing between the geometric mean of A and the geometric mean of $B$.

In the following example the points of $A$ are red and the one in $B$ is blue. The geometric median of $A$ is the red square. The green line connects the geometric means of $A$ and $B$. The black square is the geometric mean of $A \cup B$, i.e., the Fermat point of the triangle whose vertices are in $A \cup B$.

Geometric mean

  • $\begingroup$ Oh wow...how did I miss such an obvious case. Thank you! $\endgroup$
    – hLk
    Jun 7, 2019 at 1:41
  • $\begingroup$ Doing some more testing and it looks like the more points you have the closer it falls to the line. Never quite on it though $\endgroup$
    – hLk
    Jun 7, 2019 at 3:04

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