Following is a problem about prime factorization of the sum of consecutive odd primes. (source: problem 80 from The Green Book of Mathematical Problems)

Prove that the sum of two consecutive odd primes is the product of at least three (possibly repeated) prime factors.

The first thing to observe is that sum of odd numbers is even, hence the sum of two consecutive odd primes will be divisible by 2. Let’s see factorization of some of the examples:

Now let and be the consecutive odd primes, then from above observations we can conjecture that either is product of at least three distinct primes or for some odd prime such that .

To prove our conjecture, let’s assume that is NOT a product of three (or more) distinct primes (otherwise we are done). Now we will have to show that if for some odd prime then .

If then we should have . This is true since .

Now let . Since (sum of odd numbers is even), we just need to show that is not possible. On the contrary, let’s assume that . Then . By arithmetic mean property, we have

But, this contradicts the fact that are consecutive primes. Hence completing the proof of our conjecture.

*This is a nice problem where we are equating the sum of prime numbers to product of prime numbers. Please let me know the flaws in my solution (if any) in the comments.*

You must be logged in to post a comment.