When your friend is not that smart at Maths.

I challenged my friend to pick 125 125 numbers out of the first 12 4 1025 124^{1025} positive integers so that any of the two numbers x x and y y in those 125 125 numbers satisfy x 1025 y 1025 1 |\sqrt[1025]x - \sqrt[1025]y| \ge 1 .

Is it possible for him to do it?


This is part of the series: " It's easy, believe me! "

Yes, it is possible. No, it is not possible.

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1 solution

Kosma Kasprzak
Aug 10, 2018

Assume picking such 125 numbers is possible. Notice that each of the chosen numbers is in one of 125 intervals : 1 ; 2 1025 ) ; 2 1025 ; 3 1025 ) 12 3 1025 ; 12 4 1025 ) , { 12 4 1025 } \langle 1;2^{1025});\langle 2^{1025};3^{1025}) \cdots\langle 123^{1025};124^{1025}), \{124^{1025}\} , and from the pigeonhole principle some two chosen numbers a>b are in one interval x 1025 ; ( x + 1 ) 1025 ) \langle x^{1025};(x+1)^{1025}) for some integer x. Then a 1025 b 1025 = a 1025 b 1025 < ( x + 1 ) 1025 1025 x 1025 1025 = 1 |\sqrt[1025]a-\sqrt[1025]b| = \sqrt[1025]a-\sqrt[1025]b < \sqrt[1025]{(x+1)^{1025}}-\sqrt[1025]{x^{1025}}=1 . That is a contradiction and thus 125 numbers with conditions from the exercice can't be picked.

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