In 1742, Christian Goldbach wrote to Leonhard Euler proposing a simple conjecture: every even number greater than 2 is the sum of two primes (e.g., 18 = 13 5, 74 = 43 31).
In 1938, Nils Pipping verified this up to 100,000 by hand. Since then, computers have checked it for vast ranges—millions, billions, even trillions—without a single counterexample.
Yet, no general proof exists. The statement remains one of mathematics’ most famous unsolved problems known as Goldbach conjecture.