6. Verify Euler's Theorem for n = 16 and a = 5. (7 from 6.5)

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### Verification of Euler's Theorem

**Problem Statement:**
Verify Euler's Theorem for \( n = 16 \) and \( a = 5 \). (7 from 6.5)

Euler's Theorem states that for any integer \( a \) and \( n \) that are coprime (i.e., gcd(a, n) = 1), it holds that:
\[ a^{\phi(n)} \equiv 1 \ (\text{mod} \ n) \]

Here, \( \phi(n) \) is Euler's totient function, which counts the number of integers up to \( n \) that are relatively prime to \( n \).

#### Steps to Verify the Theorem:

1. **Calculate \( \phi(n) \)**:
   For \( n = 16 \):
   - Since 16 is a power of a prime (i.e., \( 16 = 2^4 \)), we can use the formula for Euler's totient function for powers of primes:
     \[ \phi(2^k) = 2^k \left(1 - \frac{1}{2}\right) = 2^k \cdot \frac{1}{2} = 2^{k-1} \]
   - Here, \( k = 4 \):
     \[ \phi(16) = 2^{4-1} = 2^3 = 8 \]
   
2. **Verify the modular equivalence**:
   For \( a = 5 \):
   - We need to check if:
     \[ 5^8 \equiv 1 \ (\text{mod} \ 16) \]
   
   Let's calculate \( 5^8 \mod 16 \):
   - Calculate \( 5^2 \):
     \[ 5^2 = 25 \]
   - Find \( 25 \mod 16 \):
     \[ 25 \equiv 9 \ (\text{mod} \ 16) \]
   - Calculate \( 5^4 \):
     \[ 5^4 = (5^2)^2 = 25^2 = 625 \]
   - Find \( 625 \mod 16 \):
     \[ 625 \mod 16 = 625 - 39 \times 16 = 625 - 624 = 1
Transcribed Image Text:### Verification of Euler's Theorem **Problem Statement:** Verify Euler's Theorem for \( n = 16 \) and \( a = 5 \). (7 from 6.5) Euler's Theorem states that for any integer \( a \) and \( n \) that are coprime (i.e., gcd(a, n) = 1), it holds that: \[ a^{\phi(n)} \equiv 1 \ (\text{mod} \ n) \] Here, \( \phi(n) \) is Euler's totient function, which counts the number of integers up to \( n \) that are relatively prime to \( n \). #### Steps to Verify the Theorem: 1. **Calculate \( \phi(n) \)**: For \( n = 16 \): - Since 16 is a power of a prime (i.e., \( 16 = 2^4 \)), we can use the formula for Euler's totient function for powers of primes: \[ \phi(2^k) = 2^k \left(1 - \frac{1}{2}\right) = 2^k \cdot \frac{1}{2} = 2^{k-1} \] - Here, \( k = 4 \): \[ \phi(16) = 2^{4-1} = 2^3 = 8 \] 2. **Verify the modular equivalence**: For \( a = 5 \): - We need to check if: \[ 5^8 \equiv 1 \ (\text{mod} \ 16) \] Let's calculate \( 5^8 \mod 16 \): - Calculate \( 5^2 \): \[ 5^2 = 25 \] - Find \( 25 \mod 16 \): \[ 25 \equiv 9 \ (\text{mod} \ 16) \] - Calculate \( 5^4 \): \[ 5^4 = (5^2)^2 = 25^2 = 625 \] - Find \( 625 \mod 16 \): \[ 625 \mod 16 = 625 - 39 \times 16 = 625 - 624 = 1
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