On 7 June 1742, the Prussian mathematician Christian Goldbach wrote a letter to Leonhard Euler  in which he suggested the following conjecture, which would later be called Goldbach's strong conjecture:
- Every integer greater than 2 can be written as the sum of two primes.
He considered 1 to be a prime number, a convention subsequently abandoned. A weaker version of Goldbach's original conjecture is:
- Every integer greater than 5 can be written as the sum of three primes.
This is called Goldbach's weak conjecture. Euler, becoming interested in the problem, wrote back to Goldbach saying that the weak conjecture would be implied by Goldbach's strong conjecture, saying that he was certain that the theorem was true ("ein ganz gewisses Theorema"), but he was unable to prove it.
- Goldbach's conjecture, part of Chris Caldwell's Prime Pages.