If you look along the possible values for a column, there may be only one
possible place in that row where a given number can be. Becuase each row
has to have all of the numbers in once and once only, if there is only one
place where that number can be, then it has to be there. The same is true
for rows and blocks, where a number only appears once, it has to go.
4
7
9
8
9
5
1
2
3
2
6
3
5
7
8
5
9
7
8
4
2
1
3
6
4
7
3
5
6
6
2
9
Example 1: Hidden single numbers
Looking at the highlighted cell in column 1. There are no other places
in that column where we can place a 4, because:
There is a 4 in the top left 3x3 block
There is a 4 in the bottom left 3x3 block
There is a 4 in column 2
Therefore the only possible place we can put a 4 in column 1, is in [r6,c1].
