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4 end points (a,b,c,d say) are chosen uniformly randomly and connected a to b and c to d by two geodesics on the 2-dim round sphere. Here uniformly is in the obvious sense of the volume form associated with the round metric. What is the probability the two curves cross?

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Perhaps the question is about the shortest geodesics, in which case the answer is not so immediate, though not very hard either. –  Wlodek Kuperberg Jan 6 at 15:41

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up vote 8 down vote accepted

The probability is 1/8. With each (generic) pair of geodesic segments you can associate another 15 pairs which you get by replacing each of the 4 end points by its antipodal point. You can easily check that of these 16 pairs, exactly 2 intersect.

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If you extend to complete circular geodesics then the probability is 1. So it is just a matter of finding the probability that the point of intersection is in both segments chosen, which is an easier linear (not spherical) problem.

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