The Goldbach Conjecture

In 1742, amateur mathematician Christian Goldbach (1690–1764) wrote to Euler about his discovery that every even integer between 4 and 100 was the sum of two primes, and asked if Euler could find a proof for every integer. Neither Euler nor anyone else was able to prove it, and the statement "Every even integer greater than two is a sum of two prime numbers" is now known as the "Goldbach Conjecture".