Look at the 8x8 grid below at left. In the rows and columns there are repeated numbers. Erasing 19 of them, we achieve the grid at right, that has no repeated numbers in any row, in any column.

+---+---+---+---+---+---+---+---+       +---+---+---+---+---+---+---+---+
| 5 | 7 | 1 | 2 | 5 | 4 | 4 | 3 |       |   | 7 | 1 |   | 5 |   | 4 | 3 |
+---+---+---+---+---+---+---+---+       +---+---+---+---+---+---+---+---+
| 4 | 3 | 1 | 2 | 7 | 5 | 6 | 3 |       | 4 | 3 |   | 2 | 7 | 5 | 6 |   |
+---+---+---+---+---+---+---+---+       +---+---+---+---+---+---+---+---+
| 5 | 5 | 3 | 4 | 2 | 1 | 7 | 8 |       |   | 5 | 3 |   | 2 |   | 7 | 8 |
+---+---+---+---+---+---+---+---+       +---+---+---+---+---+---+---+---+
| 6 | 6 | 2 | 7 | 3 | 3 | 3 | 1 |       | 6 |   | 2 | 7 |   | 3 |   | 1 |
+---+---+---+---+---+---+---+---+       +---+---+---+---+---+---+---+---+
| 3 | 2 | 5 | 6 | 9 | 1 | 8 | 6 |       | 3 | 2 | 5 |   | 9 | 1 | 8 | 6 |
+---+---+---+---+---+---+---+---+       +---+---+---+---+---+---+---+---+
| 2 | 1 | 3 | 4 | 6 | 2 | 5 | 2 |       |   | 1 |   | 4 | 6 |   | 5 | 2 |
+---+---+---+---+---+---+---+---+       +---+---+---+---+---+---+---+---+
| 9 | 8 | 4 | 1 | 4 | 6 | 2 | 3 |       | 9 | 8 | 4 | 1 |   | 6 | 2 |   |
+---+---+---+---+---+---+---+---+       +---+---+---+---+---+---+---+---+
| 7 | 5 | 6 | 5 | 8 | 5 | 1 | 4 |       | 7 |   | 6 | 5 | 8 |   | 1 | 4 |
+---+---+---+---+---+---+---+---+       +---+---+---+---+---+---+---+---+

Do the same with this 8x8 grid, erasing the minimum number of squares.

                    +---+---+---+---+---+---+---+---+
                    | 8 | 4 | 6 | 5 | 3 | 5 | 7 | 4 |
                    +---+---+---+---+---+---+---+---+
                    | 6 | 5 | 5 | 4 | 7 | 8 | 3 | 1 |
                    +---+---+---+---+---+---+---+---+ 
                    | 5 | 7 | 2 | 5 | 5 | 4 | 8 | 7 |
                    +---+---+---+---+---+---+---+---+
                    | 8 | 6 | 5 | 3 | 2 | 5 | 4 | 4 |
                    +---+---+---+---+---+---+---+---+
                    | 3 | 8 | 1 | 4 | 8 | 6 | 5 | 2 |
                    +---+---+---+---+---+---+---+---+ 
                    | 5 | 3 | 7 | 6 | 4 | 2 | 2 | 2 |
                    +---+---+---+---+---+---+---+---+
                    | 5 | 8 | 7 | 7 | 6 | 2 | 1 | 3 |
                    +---+---+---+---+---+---+---+---+
                    | 1 | 1 | 3 | 7 | 6 | 4 | 6 | 8 |
                    +---+---+---+---+---+---+---+---+ 

This type of puzzle is known as "HITORI", and you all can find a lot of them in the site www.janko.at.

I removed one restriction that it has, to make it more simpler to be solved: "each number that remains must be connected horizontally or vertically to another one". Considering this restriction, the minimum number of squares that must be erased in this one is 20. 

I do not know, and in fact I never search for, how to obtain the minimum number of cells that must be erased.

Those who may be interested, now have the guide to look for some more informations on this type of puzzles.

 

Posted by pcbouhid on 2008-08-20 22:09:27