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Let a and b be positive integers, with a<b<2a. Then, given more than
half of the integers in the set {1,2,...,a+b}, some two of the given
integers differ by a or by b.
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The proof is by William A McWorter Jr..
Form the array below.
| 1 | 2 | 3 | ... | b | b+1 | b+2 | b+3 | ... | a+b |
| a+1 | a+2 | a+3 | ... | a+b | 1 | 2 | 3 | ... | a |
There are a+b columns and each integer appears exactly twice in the
array. Now, given more than half the integers in {1,2,...,a+b}, the given
integers appear in the above array, counting multiplicity, more than a+b times.
Hence their must be two occurrences of the given integers in the same column.
Thus those occurrences differ by a or b.
The reason for the condition b<2a is that
you need only a little over a third of the integers if b=2a. And the original proof
still works if b>2a.

Copyright © 1996-2008 Alexander Bogomolny
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