Another way to discover numbers is to cross by row and column. This consists of focusing attention on a square and checking which numbers can go in that position, eliminating those that are in the same row or column.
In the following image we can verify that in the indicated box only the 7 can go, because the numbers 1,8, 3, 6 and 9 are in the same column and the numbers 2, 4 and 5 in the same row.
An improvement of this technique is achieved by also controlling the numbers that are in the same quadrant. In the following example we can see that using the cross between rows and columns we would have the candidates numbers 5, 7 and 8 to place them in the marked box. Since the numbers 5 and 8 are already placed in their positions within the quadrant, we can discard them, so the number 7 is the one that occupies the indicated position.
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