# A question about Jung's theorem

A theorem of Jung states that, given n pairwise distinct points in the Euclidean plane E, there is a unique circle of smallest radius in E that contains all the points and its radius is less than or equal to d/(3^(0.5)) where d is the maximum distance between any pair of the points. Is there an analogous theorem that applies to the 2-dimensional surface of a sphere in 3-dimensional Euclidean space?

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It seems that the problem is open. See Ю.Д.Бураго. ."Задача о круге Юнга для сферы" Математическое просвещение (II). № 6, math.ru/lib/book/djvu/mp2/mp2-6.djvu – Anton Petrunin Jul 21 '11 at 9:45
Yet one more ref: Dekster, B. V. The Jung theorem for spherical and hyperbolic spaces. Acta Math. Hungar. 67 (1995), no. 4, 315--331. – Anton Petrunin Jul 21 '11 at 10:29

Triangle with side $d$ is the worst example for $d< \arccos(-\tfrac13)$. In this case you also have uniqueness.
For $d=\arccos(-\tfrac13)$, the four point set with pairwise distance $\arccos(-\tfrac13)$ is the worse; no uniqueness and $r=d$.
It is not clear for me what happens for $d>\arccos(-\tfrac13)$. I guess $r=d$, but I do not see the worse configuration except for particular values of $d$. For example, concentric circles with radii $$2\alpha, 4\alpha, \dots, 2k\alpha$$ give the worse configuration for $\alpha=\pi/(2k+1)$, $k$ is a positive integer and $d=\pi-\alpha$.
For the regular tetrahedral configuration ($d=\cos^{-1}(-\frac{1}{3})$), surely it is impossible to cover all points by a single circle! – Victor Protsak Jul 19 '11 at 21:09