1Definition: A candidate that appears in only one cell of a Lane or a Block.
2Look For: Single candidates in cells ("Naked") or single occurences of candidates in a row, column, or Block when other candidates exist in the same cell ("Hidden.")
3Consequence: The single in the cell can be converted to a Big Number.
4Why it Works: Single Candidates are really the same thing as Single Possibilities, and work for the same reason: they are the only place the number can go (whether that is based on Lane or Block.)
Singles: Naked at C4, Hidden in row A (the 5 and the 9)
6Naked Singles are easy to spot: just look for a lone candidate in a cell. Then, enter its corresponding Big Number.
7In this example we have already solved 13 cells via the easy techniques. We have just Generated All Candidates, and now we spot a Naked Single at C4: a lone 1.
9Notice how when you entered the Big Number 1 in C4 it created two new Naked Singles: a 5 at A4, and a 9 at C6.
10Hidden Singles are really the same thing as Naked Singles, it's just that there are other candidates in the cell, obfuscating the single. However, you can still spot the Single by seeing that the candidate does not exist anywhere else in one or more of the cell's Row, Column or Block. In this example there are several Hidden Singles.
11Take a look at row 3. Since the candidates are in the same position in each cell it makes it easier to look across the row and see that there are several 3's in that row. Likewise, we can see a couple of 4's; three 5's; a couple of 6's, and several 9's. However, there is only one 7: on H3. So, we can ignore everything else in that cell, and treat it as a single 7: entering a Big Number 7 in H3..
12Next, let's look down column A. Here we see multiple 3's, 5's, 8's, and 9's. However, there is only one 6: at A8. So, we enter a Big Number 6 at A8.
14Good. Entering the 5 in F8 forces a solve of E8 in two ways: First, it erased the hard-candidate 2 from F8, forcing the remaining hard-candidate 2 in E8 to become a Big Number. Second, it has made the 2 in E8 a Hidden Single. So, we could enter the Big Number 2 in E8 now -- but we will skip doing this for now, and move on to another Hidden Single.
15All of the Hidden Single examples so far have been the lone candidate in all directions (Row, Column, and Block.) But this is not always the case. Our last example is a Hidden Single which is unique only to the column it lives on. Although it exists multiple times in the cell's Row and Block, it is still unique to its column, and so can be converted to a Big Number. Find the Hidden Single in column G, then hover over the cell and enter the Big Number.
16Good job! Remember that in SueDoku you can use the Highlight Numbers tool to help you spot singles (especially Naked Singles!)
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