The closed ball with center and radius is defined by
Croft, H. T.; Falconer, K. J.; and Guy, R. K. Unsolved Problems in Geometry. New York: Springer-Verlag, p. 1, 1991.
Weisstein, Eric W. "Closed Ball." From MathWorld--A Wolfram Web Resource. http://mathworld.wolfram.com/ClosedBall.html
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