In 1833, Heinrich Scherk conjectured that every prime of odd rank (accepting 1 as prime) can be composed by adding and subtracting all the smaller primes, each taken once. For instance, 13 is the 7th prime and 13 = 1 + 2 – 3 – 5 + 7 + 11.

In 1967 J.L. Brown Jr. proved that this is true.