See also
Ball, Closed Disk, Hypersphere, Open Disk, Open Set, Sphere
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References
Croft, H. T.; Falconer, K. J.; and Guy, R. K. Unsolved Problems in Geometry. New York: Springer-Verlag, p. 1, 1991.
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Cite this as:
Weisstein, Eric W. "Open Ball." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/OpenBall.html