Prove that essays (a) If A is open and B is closed, then A - B is open; (b) If A is closed and B is open, then A - B is closed.

Advanced Engineering Mathematics
10th Edition
ISBN:9780470458365
Author:Erwin Kreyszig
Publisher:Erwin Kreyszig
Chapter2: Second-order Linear Odes
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### Problem Statement:

**Prove that:**

(a) If \( A \) is open and \( B \) is closed, then \( A - B \) is open;

(b) If \( A \) is closed and \( B \) is open, then \( A - B \) is closed.

### Explanation:

This problem involves concepts from topology, specifically dealing with the sets and their properties in terms of openness and closedness. In this context, the subtraction \( A - B \) refers to the set difference, which is the set of elements in \( A \) that are not in \( B \).

### Detailed Explanation:

- **Open Set**: A set is open if, roughly speaking, for every point within the set, there is a surrounding "neighborhood" that is entirely contained within the set.

- **Closed Set**: A set is closed if it contains all its limit points, or equivalently, its complement within a given space is open.

### To Prove:

1. **(a) \( A - B \) is Open**:

   - Given: \( A \) is open, \( B \) is closed.
   - Objective: Show that the difference \( A - B \) is open.
   - Strategy: Use properties of open and closed sets in a given topological space to demonstrate that removing a closed set from an open set results in an open set.

2. **(b) \( A - B \) is Closed**:

   - Given: \( A \) is closed, \( B \) is open.
   - Objective: Show that the difference \( A - B \) is closed.
   - Strategy: Use the concept of complements and properties of closed sets to show that the removal of an open set from a closed set results in a closed set.

This theoretical approach is fundamental in various branches of mathematics, including analysis and topology, and has applications in fields like functional analysis and metric space theory.
Transcribed Image Text:### Problem Statement: **Prove that:** (a) If \( A \) is open and \( B \) is closed, then \( A - B \) is open; (b) If \( A \) is closed and \( B \) is open, then \( A - B \) is closed. ### Explanation: This problem involves concepts from topology, specifically dealing with the sets and their properties in terms of openness and closedness. In this context, the subtraction \( A - B \) refers to the set difference, which is the set of elements in \( A \) that are not in \( B \). ### Detailed Explanation: - **Open Set**: A set is open if, roughly speaking, for every point within the set, there is a surrounding "neighborhood" that is entirely contained within the set. - **Closed Set**: A set is closed if it contains all its limit points, or equivalently, its complement within a given space is open. ### To Prove: 1. **(a) \( A - B \) is Open**: - Given: \( A \) is open, \( B \) is closed. - Objective: Show that the difference \( A - B \) is open. - Strategy: Use properties of open and closed sets in a given topological space to demonstrate that removing a closed set from an open set results in an open set. 2. **(b) \( A - B \) is Closed**: - Given: \( A \) is closed, \( B \) is open. - Objective: Show that the difference \( A - B \) is closed. - Strategy: Use the concept of complements and properties of closed sets to show that the removal of an open set from a closed set results in a closed set. This theoretical approach is fundamental in various branches of mathematics, including analysis and topology, and has applications in fields like functional analysis and metric space theory.
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