Integer Sets Containing No Solution to x + y = 3z

We prove that a maximum subset of {1,2, …, n} containing no solutions to x + y = 3z has $$ \left\lceil {\frac{n}{2}} \right\rceil $$ elements if n ≠ 4, thus settling a conjecture of Erdos. For n ≥ 23 the set of all odd integers less than or equal to n is the unique maximum such subset.