Exercise · ZW-OPEN-SET-IN-A-METRIC-SPACE-008 · ★★☆ Intermediate
Let \(U=B(0,1)\) in \(\mathbb{R}\). What is the largest radius \(r\) with \(B(0.9,r)\subseteq U\)?
Choose one option.
Show answer
- A. \(0.1\)
Explanation
\(B(0.9,r)=(0.9-r,0.9+r)\subseteq(-1,1)\) requires \(0.9+r\le1\), that is \(r\le0.1\). With \(0.2\) the interval would reach \(1.1\).
Review the concept: Open set in a metric space
A subset \(U\) of a metric space \((X,d)\) is open if every point \(x\in U\) is the center of some open ball \(B(x,r)\) contained in \(U\).
\[\forall x\in U\ \ \exists r>0:\ B(x,r)\subseteq U\]
More practice problems: open set in a metric space
- ZW-OPEN-SET-IN-A-METRIC-SPACE-001 In usual ℝ, which of these sets is open?
- ZW-OPEN-SET-IN-A-METRIC-SPACE-002 Let U=(0,3) in ℝ. What is the largest radius r with B(1,r)⊆ U?
- ZW-OPEN-SET-IN-A-METRIC-SPACE-003 Why is (0,1] not open in usual ℝ?
- ZW-OPEN-SET-IN-A-METRIC-SPACE-004 In ℝ^2 with the Euclidean distance, which of these sets is open?
- ZW-OPEN-SET-IN-A-METRIC-SPACE-005 In a metric space (X,d) with the discrete metric, which subsets are open?
- ZW-OPEN-SET-IN-A-METRIC-SPACE-006 Let U=\x∈ℝ: x^2<4\ with the usual distance. Is it open?
- ZW-OPEN-SET-IN-A-METRIC-SPACE-007 What is bigcap_n≥1(-tfrac1n,tfrac1n) in ℝ?
- ZW-OPEN-SET-IN-A-METRIC-SPACE-009 Is ℚ an open subset of usual ℝ?
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