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Combinatorics Difficulty 6.7 National olympiad Prove it Silk Road Mathematics Competition

An integer is assigned to each unit square of a finite set of unit squares on an infinite chessboard such that the sum of numbers lying on each row as well as each column is divisible by 20022002. Prove that every number aa can be replaced by some number aa' divisible by 20022002 such that aa<2002|a - a'| < 2002 and the sums of rows and columns remain unchanged.

Solution

Consider any number a1a_1 which is not divisible by 20022002. Then there exists a number a2a_2 not divisible by 20022002 lying on the same row with a1a_1. In turn, there exists a number a3a_3 not divisible by 20022002 lying on the same column with a2a_2. We continue this process till we first reach a number aka_k which lies on the same row or column with some number ala_l, 1lk21 \le l \le k-2.

Take ll as a maximal number with this property. One easily sees that the collection CC of numbers al,al+1,,aka_l, a_{l+1}, \dots, a_k includes an even number of elements.

For each number aia_i, likl \le i \le k set d(i)=minkZai2002kd(i) = \min_{k \in \mathbb{Z}} |a_i - 2002k|.

Take m{l,,k}m \in \{l, \dots, k\} with d(m)d(i)d(m) \le d(i), likl \le i \le k and replace ama_m with the closest integer number ama'_m which is divisible by 20022002. By definition, d(m)=amam<2002d(m) = |a_m - a'_m| < 2002. Set amam=da_m - a'_m = d and replace the remaining elements in CC as follows: if im|i - m| is even, replace aia_i by aida_i - d; if im|i - m| is odd, replace aia_i by ai+da_i + d.

It can be easily seen that after these replacements the number of integers not divisible by 20022002 is decreased at least by one, the sums of rows and columns are unchanged, and if 2002k<aj<2002(k+1)2002k < a_j < 2002(k+1) then 2002k<aj<2002(k+1)2002k < a'_j < 2002(k+1). Thus, we can reach a desired configuration repeating this procedure finitely many times.

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