3. Show that Q has no subgroup isomorphic to Z2 × Z2.

Elements Of Modern Algebra
8th Edition
ISBN:9781285463230
Author:Gilbert, Linda, Jimmie
Publisher:Gilbert, Linda, Jimmie
Chapter4: More On Groups
Section4.6: Quotient Groups
Problem 11E: Find all homomorphic images of the quaternion group.
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(Q is quaternion group)

**Problem 3:**

Show that \( \mathbb{Q} \) has no subgroup isomorphic to \( \mathbb{Z}_2 \times \mathbb{Z}_2 \).

### Explanation:
- \( \mathbb{Q} \) represents the group of rational numbers under addition.
- \( \mathbb{Z}_2 \) denotes the cyclic group of order 2.
- \( \mathbb{Z}_2 \times \mathbb{Z}_2 \) is the direct product of \(\mathbb{Z}_2\) with itself, which is known to be an abelian group of order 4 where every element has order 2.

**Objective:**
Prove that within the group \( \mathbb{Q} \), there is no subgroup that matches the structure of \( \mathbb{Z}_2 \times \mathbb{Z}_2 \).

### Solution Outline:

1. **Basic Properties:**
   - \( \mathbb{Q} \) as an additive group is divisible; for any positive integer \( n \) and any element \( q \in \mathbb{Q} \), there exists an element \( r \in \mathbb{Q} \) such that \( nr = q \).

2. **Subgroup Structure of \( \mathbb{Z}_2 \times \mathbb{Z}_2 \):**
   - Elements of \( \mathbb{Z}_2 \times \mathbb{Z}_2 \) are \((0,0), (0,1), (1,0), (1,1)\) with the property that \( 2x = 0\) for all \(x\).

3. **Approach:**
   - Assume for contradiction that there exists a subgroup \( H \subset \mathbb{Q} \) isomorphic to \( \mathbb{Z}_2 \times \mathbb{Z}_2 \).
   - Identify elements \( a, b \in \mathbb{Q} \) such that every non-identity element in \( H \) has order 2. Thus, \( 2a = 0 \) and \( 2b = 0 \), implying \( a = 0 \) and \( b = 0 \).
   - Conclude that the only solution is the trivial subgroup, which contradicts the assumed structure
Transcribed Image Text:**Problem 3:** Show that \( \mathbb{Q} \) has no subgroup isomorphic to \( \mathbb{Z}_2 \times \mathbb{Z}_2 \). ### Explanation: - \( \mathbb{Q} \) represents the group of rational numbers under addition. - \( \mathbb{Z}_2 \) denotes the cyclic group of order 2. - \( \mathbb{Z}_2 \times \mathbb{Z}_2 \) is the direct product of \(\mathbb{Z}_2\) with itself, which is known to be an abelian group of order 4 where every element has order 2. **Objective:** Prove that within the group \( \mathbb{Q} \), there is no subgroup that matches the structure of \( \mathbb{Z}_2 \times \mathbb{Z}_2 \). ### Solution Outline: 1. **Basic Properties:** - \( \mathbb{Q} \) as an additive group is divisible; for any positive integer \( n \) and any element \( q \in \mathbb{Q} \), there exists an element \( r \in \mathbb{Q} \) such that \( nr = q \). 2. **Subgroup Structure of \( \mathbb{Z}_2 \times \mathbb{Z}_2 \):** - Elements of \( \mathbb{Z}_2 \times \mathbb{Z}_2 \) are \((0,0), (0,1), (1,0), (1,1)\) with the property that \( 2x = 0\) for all \(x\). 3. **Approach:** - Assume for contradiction that there exists a subgroup \( H \subset \mathbb{Q} \) isomorphic to \( \mathbb{Z}_2 \times \mathbb{Z}_2 \). - Identify elements \( a, b \in \mathbb{Q} \) such that every non-identity element in \( H \) has order 2. Thus, \( 2a = 0 \) and \( 2b = 0 \), implying \( a = 0 \) and \( b = 0 \). - Conclude that the only solution is the trivial subgroup, which contradicts the assumed structure
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