Let S be a subset of \{1,2,3, \ldots, 1989\} such that no two members of S differ by 4 or 7 . What is the largest number of elements S can have?
Let S be a subset of \{1,2,3, \ldots, 1989\} such that no two members of S differ by 4 or 7 . What is the largest number of elements S can have?