Maths Olympiad Prep

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Combinatorics Difficulty 5.3 AIME, harder Prove it Croatia

Prove that any 2001-element subset of the set {1,2,3,,3000}\{1, 2, 3, \dots, 3000\} contains three elements such that each two of them are relatively prime.

Solution

Let SS be a set of 2001 distinct positive integers from the set {1,2,3,,3000}\{1, 2, 3, \dots, 3000\}. Let us look at 500 sets
Kj={6j+ii=1,2,3,4,5,6},j=0,1,2,,499. K_j = \{6j + i \mid i = 1, 2, 3, 4, 5, 6\}, \quad j = 0, 1, 2, \dots, 499.
There is a set with at least five elements of SS among them (since 4500<20014 \cdot 500 < 2001).
If three of these five elements are odd numbers, we found the required triple.
Otherwise, the set KjK_j contains three even and two odd numbers from SS.
These two odd numbers are relatively prime since their difference is 2 or 4.
Three even numbers from the same KjK_j 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 KjK_j is less than 6, only one of them can be divisible by prime pp, p7p \ge 7.

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