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Given any 1000 integers, some two of them
differ by, or sum to, a multiple of 1997.
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Form the 999 sets of remainders modulo 1997, {0}, {i,1997-i},
for i=1,...,998. These sets partition all remainders. Hence two of the
given integers, say x and y, have remainders which lie in one of these
sets. If x and y have the same remainder, then they differ by a
multiple of 1997. Otherwise, their remainders sum to 1997 and so x+y is
a multiple of 1997.

Copyright © 1996-2008 Alexander Bogomolny
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