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Sum Square Grid Row & Column ≠ 0 (Posted on 2010-04-29) Difficulty: 3 of 5
Each cell of a 1997x1997 square grid contains either +1 or -1, with no cell being vacant.

The product of all the numbers in the ith row, and the product of all the numbers in the ith column are respectively denoted by Ri and Ci.

               1997
Prove that  Σi=1(Ri + Ci) is always nonzero.

No Solution Yet Submitted by K Sengupta    
Rating: 2.0000 (2 votes)

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a simple proof | Comment 3 of 6 |
 

Imagine all squares having +1 as a content. The overall sum-the one called sigma- will equal 2*1997*1997  which is  2 mod 4. <br> Changing square a(i,j) to -1 will  reduce the sum in row i by 2 , and in column j by 2 , leaving the overall sum  still equal to  2 mod 4.<br><br>

 if we continue and make further changes (1 into -1) striving to reduce the overall sum and get 0,- one of the following will happen:<br>

a) both the row's  and column's sums will decrease by 2.<br>

b) both the row's  and column's sums will increase by 2.<br>

c) one sum will increase and the other decrease by 2.<br>

In all those cases the overall sum will remain 2 mod 4 and 0 mod 4 cannot be reached.<br><br>

Since any  matrix out of (1997*1997)! existing can be constructed by the process I described - no matrix with sigma=0 exists.    q.e.d.

 

**Please see my later post.

Edited on April 29, 2010, 5:29 pm
  Posted by Ady TZIDON on 2010-04-29 12:31:12

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