In 1742, historian and mathematician Christian Goldbach (1690–1764) wrote a letter to Leonhard Euler (1707–1783) in which he suggested, in effect, that every integer greater than 5 is the sum of three prime numbers. A prime number is evenly divisible only by itself and 1.
Nowadays, Goldbach’s conjecture is expressed in the following equivalent form: Every even number larger than 2 is the sum of two prime numbers.
Despite centuries of effort, no one has yet been able to prove Goldbach’s conjecture. Progress has been slow.