Compute the orders of: U(5), U(7), U(35)

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ISBN:9780470458365
Author:Erwin Kreyszig
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**Title: Computing the Orders of Sets in Modular Arithmetic**

**Objective:**  
Learn how to compute the orders of the sets \( U(5) \), \( U(7) \), and \( U(35) \).

---

**Introduction to Modular Arithmetic:**

In number theory, the set \( U(n) \) represents the group of integers less than \( n \) that are coprime with \( n \). The "order" of \( U(n) \) is the number of elements in this set, often calculated using Euler's totient function \( \phi(n) \).

**Steps to Calculate the Orders:**

1. **Understanding \( U(5) \):**
   - Euler's totient function \( \phi(n) \) gives the count of integers up to \( n \) that are coprime with \( n \).
   - For a prime number \( p \), \( \phi(p) = p - 1 \).
   - Therefore, \( \phi(5) = 4 \).

2. **Calculating \( U(7) \):**
   - Here again, \( 7 \) is a prime number. So, \( \phi(7) = 7 - 1 = 6 \).

3. **Finding \( U(35) \):**
   - If \( n \) is a product of two coprime integers \( a \) and \( b \), \( \phi(n) = \phi(a) \times \phi(b) \).
   - Since \( 35 = 5 \times 7 \) (where both 5 and 7 are primes), \( \phi(35) = \phi(5) \times \phi(7) = 4 \times 6 = 24 \).

**Conclusion:**

- The orders of the sets are \( U(5) = 4 \), \( U(7) = 6 \), and \( U(35) = 24 \).
- Understanding these fundamental theorems and functions in number theory helps in various cryptographic algorithms.

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This article provides a concise introduction to computing orders using foundational concepts in number theory.
Transcribed Image Text:**Title: Computing the Orders of Sets in Modular Arithmetic** **Objective:** Learn how to compute the orders of the sets \( U(5) \), \( U(7) \), and \( U(35) \). --- **Introduction to Modular Arithmetic:** In number theory, the set \( U(n) \) represents the group of integers less than \( n \) that are coprime with \( n \). The "order" of \( U(n) \) is the number of elements in this set, often calculated using Euler's totient function \( \phi(n) \). **Steps to Calculate the Orders:** 1. **Understanding \( U(5) \):** - Euler's totient function \( \phi(n) \) gives the count of integers up to \( n \) that are coprime with \( n \). - For a prime number \( p \), \( \phi(p) = p - 1 \). - Therefore, \( \phi(5) = 4 \). 2. **Calculating \( U(7) \):** - Here again, \( 7 \) is a prime number. So, \( \phi(7) = 7 - 1 = 6 \). 3. **Finding \( U(35) \):** - If \( n \) is a product of two coprime integers \( a \) and \( b \), \( \phi(n) = \phi(a) \times \phi(b) \). - Since \( 35 = 5 \times 7 \) (where both 5 and 7 are primes), \( \phi(35) = \phi(5) \times \phi(7) = 4 \times 6 = 24 \). **Conclusion:** - The orders of the sets are \( U(5) = 4 \), \( U(7) = 6 \), and \( U(35) = 24 \). - Understanding these fundamental theorems and functions in number theory helps in various cryptographic algorithms. --- This article provides a concise introduction to computing orders using foundational concepts in number theory.
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