Suppose B C A, B + A, and B ~ A. Prove that A is infinite.
Advanced Engineering Mathematics
10th Edition
ISBN:9780470458365
Author:Erwin Kreyszig
Publisher:Erwin Kreyszig
Chapter2: Second-order Linear Odes
Section: Chapter Questions
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Transcribed Image Text:### Problem Statement:
**9.** Suppose \( B \subseteq A \), \( B \neq A \), and \( B \sim A \). Prove that \( A \) is infinite.
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### Explanation and Solution:
**Given:**
- \( B \subseteq A \): Set \( B \) is a subset of set \( A \).
- \( B \neq A \): Set \( B \) is not equal to set \( A \); hence, \( B \) is a proper subset of \( A \).
- \( B \sim A \): Set \( B \) is equivalent to set \( A \), meaning there exists a bijection (one-to-one and onto function) between \( B \) and \( A \).
**Objective:**
- Prove that set \( A \) is infinite.
**Proof:**
To prove that \( A \) is infinite, assume that \( A \) is finite and derive a contradiction.
1. **Assumption (for contradiction)**: Suppose \( A \) is finite.
- This implies that \( A \) has a finite number of elements, say \( n \).
2. **Proper subset implication**:
- Since \( B \subseteq A \) and \( B \neq A \), \( B \) must have fewer elements than \( A \), i.e., the number of elements in \( B \) is less than \( n \).
3. **Cardinality argument**:
- If \( B \) is a proper subset of \( A \) and \( A \) is finite, the number of elements in \( B \) must be \( m \), where \( 0 \leq m < n \).
4. **Contradiction with equivalence**:
- Given \( B \sim A \), there exists a bijection between \( B \) and \( A \).
- For finite sets, a bijection requires both sets to have the same number of elements for one-to-one correspondence.
- Hence, \( B \) should have \( n \) elements if it is equivalent to \( A \), but \( B \) has only \( m \) elements (\( m < n \)), which is a contradiction.
5. **Conclusion**:
- Therefore, the assumption that set \( A \) is finite leads to a contradiction.
- Hence, \( A \) must
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