1. The hidden singleton technique

# The hidden singleton technique

Hello and welcome to this course on the hidden singleton technique!

The hidden singleton technique is a very challenging and fun technique to use. It consists of using elimination to guess the number in a cell.

In this course, we will see how to apply this technique and put it into practice through several examples. Very good course!

## Application of the hidden singleton technique

A hidden singleton means that there is only one possible cell for a number in a specific house.

The number is hidden among the other possible numbers for that column.

Let's take a look at the case in this Sudoku grid and try to see which cells in this house might contain the number 1.

Let's focus on line number 8.

Cell R8-C4 already has the number 1, this means that there can be no other 1 numbers on this line.

Now that we have been able to eliminate line 8, let's look at line 9.

We can see that cell R9-C3 also has the number 1, this means that there can be no other 1 numbers on this line.

Finally, when we look at column number 7, we can see that cell R4-C7 also has the number 1. Therefore, this number cannot be added in this column.

When we retrace the previous path and focus on cell number 9, we see that there is only one cell left where the 1 can appear. And, this cell is none other than cell R7-C9

## Another example of hidden singleton technique

In the previous example, we saw how to start proceeding by elimination thanks to the columns and rows. In this example, we will see how to proceed by elimination thanks to the cells.

Let's look at row 4 on this grid and see which cells could contain the number 5 in this house.

Cell R8-C4, found in cell number 6 contains the number 5.

There can be no other 5 anywhere in this cell, so since there is only one cell left that does not belong to the same house as cell R8-C4. Therefore, it is cell R4-C6 that must contain the number 5

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