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Source: INMO 2001 Problem 4

October 10, 2005
modular arithmeticcombinatorics unsolvedcombinatorics

Problem Statement

Show that given any nine integers, we can find four, a,b,c,da, b, c, d such that a+b−c−da + b - c - dis divisible by 2020. Show that this is not always true for eight integers.
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