The set is often written as B_{r}(p), meaning the ball of radius r centred on a point p. This is defined as:
If the lessthan symbol (<) is replaced by a lessthanorequalto (≤), the above definition becomes that of a closed ball. This is usually notated by adding an overscore[?] to the B.
If r = 1, then it is called a unit ball.
A set is bounded if it is contained in a ball.
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