9. Show that there are infinitely many primes of the form 16k + 1. More generally, show that for any r > 0, there are infinitely many primes of the form 2" k + 1. 10

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Can you do #9?
### Exercises for Section 4.1

1. **Use Fermat's Theorem to compute the following quantities:**
   - (a) \( 31^{100} \mod 19 \)
   - (b) \( 2^{10000} \mod 29 \)
   - (c) \( 9^{999} \mod 31 \)

2. **Show that** \( 11^{84} - 5^{84} \) **is divisible by 7.**

3. **Show that if** \( n \equiv 2 \pmod{4} \), **then** \( 9^n + 8^n \) **is divisible by 5.**

4. **For which values of** \( n \) **is** \( 3^n + 2^n \) **divisible by 13? By 7?**

5. **Use Fermat's Theorem to show that** \( n^{13} - n \) **is divisible by 2730 for all** \( n \).

6. **Show that if** \( p > 3 \) **is prime, then** \( ab^p - ba^p \) **is divisible by** \( 6p \).

7. **Show, using the Binomial Theorem, that if** \( p \) **is prime and** \( a \) **and** \( b \) **are integers, then** \( (a+b)^p \equiv a^p + b^p \pmod{p} \).

8. **Show that no prime number of the form** \( 4k + 3 \) **can divide a number of the form** \( n^2 + 1 \).

9. **Show that there are infinitely many primes of the form** \( 16k + 1 \). **More generally, show that for any** \( r > 0 \), **there are infinitely many primes of the form** \( 2^rk + 1 \).

10. **Let** \( n = r^4 + 1 \). **Show that** \( 3, 5, \) **and** \( 7 \) **cannot divide** \( n \). **What is the smallest prime that can divide** \( n \)? **Determine the form of the prime divisors of** \(
Transcribed Image Text:### Exercises for Section 4.1 1. **Use Fermat's Theorem to compute the following quantities:** - (a) \( 31^{100} \mod 19 \) - (b) \( 2^{10000} \mod 29 \) - (c) \( 9^{999} \mod 31 \) 2. **Show that** \( 11^{84} - 5^{84} \) **is divisible by 7.** 3. **Show that if** \( n \equiv 2 \pmod{4} \), **then** \( 9^n + 8^n \) **is divisible by 5.** 4. **For which values of** \( n \) **is** \( 3^n + 2^n \) **divisible by 13? By 7?** 5. **Use Fermat's Theorem to show that** \( n^{13} - n \) **is divisible by 2730 for all** \( n \). 6. **Show that if** \( p > 3 \) **is prime, then** \( ab^p - ba^p \) **is divisible by** \( 6p \). 7. **Show, using the Binomial Theorem, that if** \( p \) **is prime and** \( a \) **and** \( b \) **are integers, then** \( (a+b)^p \equiv a^p + b^p \pmod{p} \). 8. **Show that no prime number of the form** \( 4k + 3 \) **can divide a number of the form** \( n^2 + 1 \). 9. **Show that there are infinitely many primes of the form** \( 16k + 1 \). **More generally, show that for any** \( r > 0 \), **there are infinitely many primes of the form** \( 2^rk + 1 \). 10. **Let** \( n = r^4 + 1 \). **Show that** \( 3, 5, \) **and** \( 7 \) **cannot divide** \( n \). **What is the smallest prime that can divide** \( n \)? **Determine the form of the prime divisors of** \(
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