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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#4. Thanks.

### Problem Statement

4. Suppose that \( G \) is a finite group with an element \( g \) of order 5 and an element \( h \) of order 7. Why must \( |G| \geq 35 \)? (1 from 6.5)

### Explanation

Given:
- \( G \) is a finite group.
- \( g \) is an element in \( G \) with order 5.
- \( h \) is an element in \( G \) with order 7.

To show:
- The order of \( G \), \( |G| \), must be at least 35.

### Solution Sketch

1. **Basic Concepts**: 
    - The **order of an element** in a group is the smallest positive integer \( n \) such that \( g^n = e \), where \( e \) is the identity element.
    - By **Lagrange's Theorem**, the order of any element in a finite group divides the order of the group.

2. **Orders and Multiples**:
    - Given \( g \) has an order of 5, \( 5 \) divides \( |G| \).
    - Given \( h \) has an order of 7, \( 7 \) divides \( |G| \).

3. **Relatively Prime Orders**:
    - The greatest common divisor (gcd) of 5 and 7 is 1, since they are relatively prime.

4. **Conclusion**:
    - Since 5 and 7 are both divisors of \( |G| \) and are relatively prime, their least common multiple must divide \( |G| \).
    - The least common multiple (LCM) of 5 and 7 is \( 35 \).

Thus,
\[ |G| \text { must be a multiple of 35, and therefore } |G| \geq 35 \]

This completes the proof that \( |G| \geq 35 \).
Transcribed Image Text:### Problem Statement 4. Suppose that \( G \) is a finite group with an element \( g \) of order 5 and an element \( h \) of order 7. Why must \( |G| \geq 35 \)? (1 from 6.5) ### Explanation Given: - \( G \) is a finite group. - \( g \) is an element in \( G \) with order 5. - \( h \) is an element in \( G \) with order 7. To show: - The order of \( G \), \( |G| \), must be at least 35. ### Solution Sketch 1. **Basic Concepts**: - The **order of an element** in a group is the smallest positive integer \( n \) such that \( g^n = e \), where \( e \) is the identity element. - By **Lagrange's Theorem**, the order of any element in a finite group divides the order of the group. 2. **Orders and Multiples**: - Given \( g \) has an order of 5, \( 5 \) divides \( |G| \). - Given \( h \) has an order of 7, \( 7 \) divides \( |G| \). 3. **Relatively Prime Orders**: - The greatest common divisor (gcd) of 5 and 7 is 1, since they are relatively prime. 4. **Conclusion**: - Since 5 and 7 are both divisors of \( |G| \) and are relatively prime, their least common multiple must divide \( |G| \). - The least common multiple (LCM) of 5 and 7 is \( 35 \). Thus, \[ |G| \text { must be a multiple of 35, and therefore } |G| \geq 35 \] This completes the proof that \( |G| \geq 35 \).
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