Jung's theorem explained
In geometry, Jung's theorem is an inequality between the diameter of a set of points in any Euclidean space and the radius of the minimum enclosing ball of that set. It is named after Heinrich Jung, who first studied this inequality in 1901. Algorithms also exist to solve the smallest-circle problem explicitly.
Statement
Consider a compact set
and let
be the diameter of K, that is, the largest Euclidean distance between any two of its points. Jung's theorem states that there exists a closed ball with radius
}
that contains K. The boundary case of equality is attained by the regular n-simplex.
Jung's theorem in the plane
The most common case of Jung's theorem is in the plane, that is, when n = 2. In this case the theorem states that there exists a circle enclosing all points whose radius satisfies
},
and this bound is as tight as possible since when K is an equilateral triangle (or its three vertices) one has
}.
General metric spaces
in any
metric space,
. The first inequality is implied by the
triangle inequality for the center of the ball and the two diametral points, and the second inequality follows since a ball of radius
centered at any point of
will contain all of
. Both these inequalities are tight:
- In a uniform metric space, that is, a space in which all distances are equal,
.
: any two closed balls of radius
centered at points of
have a non-
empty intersection, therefore all such balls have a common intersection, and a radius
ball centered at a point of this intersection contains all of
.
Versions of Jung's theorem for various non-Euclidean geometries are also known (see e.g. Dekster 1995, 1997).
References
- Katz, M. . Jung's theorem in complex projective geometry . Quart. J. Math. Oxford . 36 . 4 . 1985 . 451–466 . 10.1093/qmath/36.4.451.
- Dekster, B. V. . The Jung theorem for the spherical and hyperbolic spaces . . 67 . 4 . 1995 . 315–331 . 10.1007/BF01874495 .
- Dekster, B. V. . The Jung theorem in metric spaces of curvature bounded above . . 125 . 8 . 1997 . 2425–2433 . 10.1090/S0002-9939-97-03842-2. free .
- Jung, Heinrich . Über die kleinste Kugel, die eine räumliche Figur einschließt . J. Reine Angew. Math. . 123 . 1901 . 241–257 . de.
- Jung, Heinrich . Über den kleinsten Kreis, der eine ebene Figur einschließt . J. Reine Angew. Math. . 137 . 1910 . 310–313 . de.
- Book: Rademacher, Hans . Toeplitz, Otto . The Enjoyment of Mathematics . 1990 . Dover . 978-0-486-26242-0 . true . chapter 16.