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  • What is the maximal number of disjoint regions obtained on the sphere by dividing it with $n$ great circles?

For $n= 1$ we have $2$ regions, for $n=2 $ we have $2^2$, for $n=3$ the number is $2^3$,... what next? - what would be the best approach to this problem? How to use geometrical constraints solving it?

... and ...

  • Is it possible to find always $n$ great circles (in the case when the number of regions is maximal) for such division that areas of regions would be equal ? (I suppose not but how to prove it?)
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For the count: We use the fact that the Euler characteristic of the sphere is $2$. A generic configuration of great circles will have $ 2\times \frac {n(n-1)}2=n(n-1)$ vertices and $n\times 2(n-1)$ edges. Thus we have $$2=\#F_n -2n^2+2n+n^2-n=\#F_n-n^2+n\implies \boxed {\#F_n=n^2-n+2}$$

As a sanity check we remark that $\#F_1=2,\#F_2=4,\#F_3=9-3+2=8$ as desired.

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  • $\begingroup$ So in this case a good approach is linked with graph theory, without even defining equations for circles? $\endgroup$ – Widawensen Dec 16 '16 at 14:33
  • $\begingroup$ I wouldn't even know how to use the defining equations for circles in this case....so, yes. I'm just using basic notions about triangulations. $\endgroup$ – lulu Dec 16 '16 at 14:34
  • $\begingroup$ Fine, so it's simply a topology, however the second question is probably impossible to solve without geometry... but who knows.. "equal areas" is also a very general condition.. $\endgroup$ – Widawensen Dec 16 '16 at 14:43
  • $\begingroup$ I didn't think about the area question. Like you, my gut reaction is that it isn't possible...but i really don't know. $\endgroup$ – lulu Dec 16 '16 at 14:45

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