Exercise · ZW-OPEN-BALL-001 · ★☆☆ Basic
In \(\mathbb{R}\) with \(d(x,y)=|x-y|\), which interval is the open ball \(B(2,3)\)?
Choose one option.
Show answer
- C. \((-1,5)\)
Explanation
These are the \(y\) with \(|y-2|<3\), that is \(-1<y<5\). The endpoints are excluded because the inequality is strict.
Review the concept: Open ball
The open ball \(B(x,r)\) with center \(x\) and radius \(r>0\) in a metric space \((X,d)\) is the set of points of \(X\) whose distance to \(x\) is less than \(r\).
\[B(x,r)=\{\,y\in X : d(x,y)
More practice problems: open ball
- ZW-OPEN-BALL-002 In the usual ℝ, which of these points belongs to B(0,2)?
- ZW-OPEN-BALL-003 In Euclidean ℝ^2, which point lies in B((0,0),1)?
- ZW-OPEN-BALL-004 With the max distance d_∞ on ℝ^2, the ball B((0,0),2) is a square. What is its area?
- ZW-OPEN-BALL-005 With the taxicab distance d_1 on ℝ^2, the ball B((0,0),1) is a rhombus with vertices (pm1,0) and (0…
- ZW-OPEN-BALL-006 In a space with the discrete metric, what is the ball B(a,1)?
- ZW-OPEN-BALL-007 In the usual ℝ, what is the intersection B(0,2)∩ B(3,2)?
- ZW-OPEN-BALL-008 How many points (a,b) with integer coordinates lie in the Euclidean ball B((0,0),3)?
- ZW-OPEN-BALL-009 Let y∈ B(a,r) and s=r-d(a,y). In ℝ with a=0, r=5 and y=3, what is s, the radius ensuring B(y,s)⊆ B(…