We didn't talk about prime numbers in my class, but several of the homework problems mention them. For instance: Prove that if every even natural number greater than 2 is the sum of two primes, then every odd natural number greater than 5 is the sume of three primes. Assume that n is an even natural number and n is greater than 2. Then n=2k, where k is an integer, and 2k is greater than 2. But how do I define a prime number in a proof? Any hints/help would be appreciated!