Erdős–Turán conjecture on additive bases
The Erdős–Turán conjecture is an old unsolved problem in additive number theory (not to be confused with Erdős conjecture on arithmetic progressions) posed by Paul Erdős and Pál Turán in 1941.
It concerns additive bases, subsets of natural numbers with the property that every natural number can be represented as the sum of a bounded number of elements from the basis. Roughly, it states that the number of representations of this type cannot also be bounded.