Suppose B C A, B + A, and B ~ A. Prove that A is infinite.

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### Problem Statement:

**9.** Suppose \( B \subseteq A \), \( B \neq A \), and \( B \sim A \). Prove that \( A \) is infinite.

---

### 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
Transcribed Image Text:### Problem Statement: **9.** Suppose \( B \subseteq A \), \( B \neq A \), and \( B \sim A \). Prove that \( A \) is infinite. --- ### 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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