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Please note: This section will be more relevant to you and easier to follow when you have acquired the Sudoku Instructions program and the free course in sudoku.

To solve a sudoku puzzle you need to locate ‘single candidates’, that is squares where only one digit can be placed.

Consider this sudoku puzzle we entered from a newspaper:

To help find the ‘single candidates’ you can get the program to display the possible candidates for each square.

You just click menu item ‘Options’ and then click ‘Show Candidates’.

Then all possible candidate digits (‘the candidate table’) will be shown as seen in the next picture.

The candidate table is calculated by the program. It is updated automatically when a digit is entered.

Now you can immediately see that there are ‘naked single candidates’ (red arrows) in square E4 (3) and in square C9 (8).

(These candidates are called naked because they are not hidden among other candidates.)

You can enter digit 3 in square E4 and digit 8 in square C9 by just clicking those candidate numbers.

Then the sudoku board will look like in this picture.

Now it is not so easy to locate other single candidates. You can look at each row, column and box to see if there is a candidate digit, which occurs only once.

For example look at row 7. Is there a candidate digit, which occurs only once in that row? Actually there is. Within row 7 candidate digit 3 occurs only in square F7. Digit 3 is a ‘hidden’ single candidate as it is hidden among other candidates in that square. You can click that candidate digit to enter it on the sudoku board.

The program has a facility, which makes it easier to locate the hidden single candidates.

You click menu item ‘Options’ and then click ‘Define Candidates to be Displayed’.

Then this window appears:

Here you can select the candidates that should be displayed on the sudoku board.

If you select just digit 3 to be displayed, it becomes much easier to see that digit 3 is a single candidate for row 7 in square F7 as seen in the picture below.

If you display the candidate table with one digit at a time, it will be easier to locate the single candidates. In this way you can solve the sudoku faster.

As described in the help section the Sudoku Instructions program can find single candidates for you.

When the candidate table is displayed, the identified single candidate is shown in green bold type with the square marked with a green frame, see picture below.

This is the same sudoku puzzle as above just solved a bit more.

The program has identified digit 7 in square I1 as a hidden single candidate for row 1.

This you can also see in the comments box below the sudoku board.

(Within row 1 digit 7 cannot be inside box J or K already having a 7.

Column G also has a 7, so in row 1 digit 7 can only be in square I1.)

You click the single candidate digit 7 to place it on the sudoku board.

In difficult sudoku puzzles it may be hard to find single candidates. It may be necessary to reduce the number of candidates. The Sudoku Instructions program can help you doing just that.

We will describe a relatively easy candidate reduction method: the naked candidates method.

Take a look at the sudoku. Is there a row, column or box where only the same two candidates occur in two squares?

Actually there is. In row 5 the candidate pair 4 and 7 occurs in squares D5 and E5. No other candidates are in those two squares, they are ‘naked’ here.

Thus digits 4 and 7 form a pair of naked candidates.

This means that digit 4 and 7 must be in those two squares.

Therefore these two candidates can be removed from the other squares in row 5, that is from squares A5 (7) and B5 (4 and 7). Since the naked candidates are also confined to box N, you can also remove digit 4 from square F6.

The program gives you the possibility to remove candidates ‘manually’ so to speak.

First you must click ‘Options’ and ‘Set Manual Removal of Candidates: On’ to enable this feature as shown in this picture.

When enabled you can remove the candidates by right clicking on them.

After you have removed candidate 7 from A5, candidate 4 from F6 and candidates 4 and 7 from B5 by right clicking them the board looks like this:

This candidate reduction has led to emergence of isolated single candidates in squares F1 (5) and F6 (9). Thus you can enter those numbers in the squares.

By performing candidate reduction repeatedly, more single candidates will emerge. In this way you can solve difficult sudoku puzzles.

The Sudoku Instructions program offers also a number of other methods of candidate reduction. The program can find and display candidates that can be eliminated and it can also perform the elimination for you.

Here you can see an outline of some of the additional advanced solving features of the Sudoku Instructions program. The most important are methods for candidate reduction, which are summarized here.

All these additional features and candidate reduction methods are described in extensive detail in the free course in sudoku with special reference to the Sudoku Instructions program.

You will get the free course in sudoku as a gift, when you buy the Sudoku Instructions program.

Copyright: The Sudoku Instructions Programming Unit -