We discussed greatest common divisors (Definition 6.11), and used them to prove that the integer linear combination of two number are the multiples of their gcd (Theorem 6.12).
We then discussed exercise 6.34, proving that there are infinitely many primes. Some interesting exercises to do if you are bored: 6.41 is anotehr proof that there are infinitely many primes, 6.40 discusses perfect numbers.
On Thursday, I will discuss problem 5.19 in the previous homework.