Sudoku Techniques (Part 1)

Today’s sudoku technique follows on closely from the previous one and still counts as an easy hint for a sudoku solver. Here, we are still looking for singles but in this case they have become “hidden singles”. All this means in real terms, is that the single has other candidates in the cell alongside it.

If, however, you check the houses associated with that cell (remember house = row,column or box) one of them will make it clear that the single is the only possibility for that cell. Let’s look at a continuation of the previous grid.

Hidden Single candidates

I have filled in several other naked singles as I found them and as they were exposed (by crossing out all other instances of that number among the cell peers). As we saw before, this is a vital step after finding any number….it may bring to light other easily recognised cases for you to solve. So now, we have no more naked singles – let’s look instead at cell 3,3. In this cell we have two candidates 5 and 7, so the single is not obvious. If we look at the box containing this cell (or the row in this case) we can see that there are no other cells where 5 is a candidate. Since we must have a 5 in each house, it must live in 3,3. This leads us to the situation below, where I have removed all other 5’s from column 3. There is another hidden single in 2,7 see if you can work it out and do any others you can find.