The structure of maximum subsets of {1,...,n} with no solutions to a+b=kc

If $k$ is a positive integer, we say that a set $A$ of positive integers is $k$-sum-free if there do not exist $a,b,c$ in $A$ such that $a + b = kc$. In particular we give a precise characterization of the structure of maximum sized $k$-sum-free sets in $\{1,...,n\}$ for $k\ge 4$ and $n$ large