Exercise · ZW-SEQUENTIAL-COMPACTNESS-METRIC-SPACES-005 · ★☆☆ Basic
Which of these subsets of \(\mathbb{R}^2\), with the Euclidean distance, is sequentially compact?
Choose one option.
Show answer
- C. \(\{x^2+y^2\le1\}\)
Explanation
The closed disk is closed and bounded, hence compact by Heine-Borel and sequentially compact. The open disk misses its boundary limits, while the hyperbola and the half-plane are unbounded.
Review the concept: Sequential compactness
A metric space \(X\) is sequentially compact if every sequence \((x_n)\) in \(X\) has a subsequence \((x_{n_k})\) converging to a point \(x\in X\).
\[\forall (x_n)\subseteq X\ \ \exists\, n_1
More practice problems: sequential compactness
- ZW-SEQUENTIAL-COMPACTNESS-METRIC-SPACES-001 Which of these spaces, with the usual distance, is sequentially compact?
- ZW-SEQUENTIAL-COMPACTNESS-METRIC-SPACES-002 Why is ℝ not sequentially compact?
- ZW-SEQUENTIAL-COMPACTNESS-METRIC-SPACES-003 To prove that (0,1) is not sequentially compact, which sequence works?
- ZW-SEQUENTIAL-COMPACTNESS-METRIC-SPACES-004 In X=(1,2] with the usual distance, does the sequence x_n=1+tfrac1n have a subsequence converging i…
- ZW-SEQUENTIAL-COMPACTNESS-METRIC-SPACES-006 Let x_n=cos \!big(tfracnπ2big) in [-1,1]. Which values are limits of some subsequence?
- ZW-SEQUENTIAL-COMPACTNESS-METRIC-SPACES-007 Let x_n=(-1)^n (n)/(n+1) in [-1,1]. To which values do its convergent subsequences converge?
- ZW-SEQUENTIAL-COMPACTNESS-METRIC-SPACES-008 For a metric space X, which statement is true?
- ZW-SEQUENTIAL-COMPACTNESS-METRIC-SPACES-009 Let K be a sequentially compact metric space and F⊆ K closed. Is F sequentially compact?
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