Prove that any 2001-element subset of the set contains three elements such that each two of them are relatively prime.
Solution
Let be a set of 2001 distinct positive integers from the set . Let us look at 500 sets
There is a set with at least five elements of among them (since ).
If three of these five elements are odd numbers, we found the required triple.
Otherwise, the set contains three even and two odd numbers from .
These two odd numbers are relatively prime since their difference is 2 or 4.
Three even numbers from the same are three consecutive even numbers. Only one of them is divisible by 3 and at most one of them can be divisible by 5. Therefore we can choose an even number which is not divisible by 3 nor by 5.
That even number together with two odd numbers gives the required triple. Since the difference of the numbers from the set is less than 6, only one of them can be divisible by prime , .