I challenged my friend to pick numbers out of the first positive integers so that any of the two numbers and in those numbers satisfy .
Is it possible for him to do it?
This is part of the series: " It's easy, believe me! "
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Assume picking such 125 numbers is possible. Notice that each of the chosen numbers is in one of 125 intervals : ⟨ 1 ; 2 1 0 2 5 ) ; ⟨ 2 1 0 2 5 ; 3 1 0 2 5 ) ⋯ ⟨ 1 2 3 1 0 2 5 ; 1 2 4 1 0 2 5 ) , { 1 2 4 1 0 2 5 } , and from the pigeonhole principle some two chosen numbers a>b are in one interval ⟨ x 1 0 2 5 ; ( x + 1 ) 1 0 2 5 ) for some integer x. Then ∣ 1 0 2 5 a − 1 0 2 5 b ∣ = 1 0 2 5 a − 1 0 2 5 b < 1 0 2 5 ( x + 1 ) 1 0 2 5 − 1 0 2 5 x 1 0 2 5 = 1 . That is a contradiction and thus 125 numbers with conditions from the exercice can't be picked.