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# Sudoku-puzzles-online

General Help

This site allows you to solve and play

**9X9 Sudoku** and

** Hexadoku 16X16 Sudoku puzzles**.

You can enter your own puzzles, for example from a magazine, or select one of our 40,000 pre-set puzzles.

This solver can also help you create and test the puzzles you invent yourself...

**The site server may at any time: ** - Give you the solution of the puzzle in progress.

- Tell you and advise you on the next square to solve without giving you the solution.

- Give you the solution and the solving method for the squares that have a solution.

- For the 9 X 9 sudoku, give you all the possible candidates for your puzzle in progress.

- For the 9 X 9 sudoku, allow you to play in verbose mode: that is solve the puzzle with all the candidates automatically updated as you enter your solutions.

**Reminder:**A Sudoku is a grid of 9 X 9 squares.

Each square admits a number from 1 to 9.

A grid is composed of three types of **area** of 9 squares: 9 horizontal rows, 9 vertical columns and 9 boxes of 3 x 3 squares.

All areas are subject to the same constraint: they all contain the digits 1 to 9, consequently an area cannot allow the repetition of any number.

So below "**area of a square**" will refer to the row, column or box in which the square is included.

The server can advise you on the following four basic methods:

• Method by inclusion.

• Method by exclusion.

• Method by an exclusive pair.

• Method by multiple choice.

## Inclusive method

A square has a solution if you can include only one digit: more precisely, if the three areas of a square have already included eight different numbers then the remaining number is the solution of this square.

**Example 1: **with reference to a square, all the other squares in the row have the solutions 1, 2, 3, 4, 6, 7, 8 and 9. So 5 is the only solution for this free square.

**Example 2: **with reference to a square, the other squares in the row have the solutions 1, 2, 3 and 4 and the other squares in the column have the solutions 6, 7, 8 and 9. So 5 is the only solution for this free square.

## Exclusive method

This resolution is more difficult to perceive visually than the previous one:

A square admits a number if all the other squares in one of the 3 areas of the square excludes this number.

**Example: **A box may allow 1, 2 and 3.

On the other hand, all the other squares in the row exclude the possibility 2. So 2 is the solution of this square.

## Exclusive pair method

If two squares in an area admit the same two numbers and only two numbers then these two numbers are excluded from all other squares in this area.

**Example 1: **in one row a first square admits only 1 and 2, a second square also admits only 1 and 2 and a third square admits only 1 and 3. Then 1 will be excluded from the third square leaving 3 as the solution.

**Example 2: **in one column a first square admits only 1 and 2, a second square also admits only 1 and 2 and a third square admits only 1, 2 and 4. Then 1 and 2 will be excluded from the third square leaving 4 as the solution.

## Multiple choice method

If no square admits a single solution by "inclusion", "exclusion" or "exclusive pair" then you will be offered one of the squares with the fewest opportunities. This only occurs in the most difficult puzzles, called "fiendish", or else in puzzles admitting several solutions.

That is why if you ask for the resolution of a blank grid, you will get the equivalent of a complete random grid, with the words "multiple solutions"...