Construct a family of sets that demonstrates R is not compact

Advanced Engineering Mathematics
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ISBN:9780470458365
Author:Erwin Kreyszig
Publisher:Erwin Kreyszig
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**Title: Constructing a Family of Sets to Demonstrate Non-Compactness in \( \mathbb{R} \)**

**Introduction:**
Compactness is a fundamental concept in topology and analysis. A set is compact if every open cover has a finite subcover. This property does not hold for certain spaces, and the real number line \( \mathbb{R} \) is a classic example. Here's how you can construct a family of sets to demonstrate that \( \mathbb{R} \) is not compact.

**Understanding the Concept:**
To show that \( \mathbb{R} \) is not compact, consider constructing an open cover of \( \mathbb{R} \) that has no finite subcover. This construction involves creating a collection of open intervals that cover all real numbers but cannot be reduced to a finite number of intervals still covering all of \( \mathbb{R} \).

**Construction Example:**
Consider the family of open intervals:
\[ \mathcal{F} = \{ (n, n+2) \mid n \in \mathbb{Z} \} \]
where \( \mathbb{Z} \) is the set of all integers. Each interval \( (n, n+2) \) covers real numbers just beyond each integer point.

- **Union of the family:** The union of these sets, \( \bigcup_{n \in \mathbb{Z}} (n, n+2) \), covers the entire real line, as every real number \( x \in \mathbb{R} \) lies within some interval \( (n, n+2) \).

- **Non-existence of finite subcover:** No finite subset of this family can cover \( \mathbb{R} \) because removing intervals will leave gaps corresponding to numbers not covered by any remaining intervals.

**Conclusion:**
This example illustrates that \( \mathbb{R} \) is not compact because it does not satisfy the finite subcover condition. Understanding such properties is crucial for deeper insights into topology and the structure of real numbers.
Transcribed Image Text:**Title: Constructing a Family of Sets to Demonstrate Non-Compactness in \( \mathbb{R} \)** **Introduction:** Compactness is a fundamental concept in topology and analysis. A set is compact if every open cover has a finite subcover. This property does not hold for certain spaces, and the real number line \( \mathbb{R} \) is a classic example. Here's how you can construct a family of sets to demonstrate that \( \mathbb{R} \) is not compact. **Understanding the Concept:** To show that \( \mathbb{R} \) is not compact, consider constructing an open cover of \( \mathbb{R} \) that has no finite subcover. This construction involves creating a collection of open intervals that cover all real numbers but cannot be reduced to a finite number of intervals still covering all of \( \mathbb{R} \). **Construction Example:** Consider the family of open intervals: \[ \mathcal{F} = \{ (n, n+2) \mid n \in \mathbb{Z} \} \] where \( \mathbb{Z} \) is the set of all integers. Each interval \( (n, n+2) \) covers real numbers just beyond each integer point. - **Union of the family:** The union of these sets, \( \bigcup_{n \in \mathbb{Z}} (n, n+2) \), covers the entire real line, as every real number \( x \in \mathbb{R} \) lies within some interval \( (n, n+2) \). - **Non-existence of finite subcover:** No finite subset of this family can cover \( \mathbb{R} \) because removing intervals will leave gaps corresponding to numbers not covered by any remaining intervals. **Conclusion:** This example illustrates that \( \mathbb{R} \) is not compact because it does not satisfy the finite subcover condition. Understanding such properties is crucial for deeper insights into topology and the structure of real numbers.
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