Jung's Theorem

Every finite set of points with Span $d$ has an enclosing Circle with Radius no greater than $\sqrt{3}\,d/3$.

References

Le Lionnais, F. Les nombres remarquables. Paris: Hermann, p. 28, 1983.

Rademacher, H. and Toeplitz, O. The Enjoyment of Mathematics: Selections from Mathematics for the Amateur. Princeton, NJ: Princeton University Press, pp. 103-110, 1957.