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responsibility for the correctness of any of the information on this
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## Background

The name SuDoku is Japanese and can roughly be translated as "single
number". The game SuDoku has been popular in Japan since the mid 1980s
but has only recently become popular in the rest of the world, triggered
primarily by the publishing of SuDoku puzzles in British newspapers
starting November 2004.

Although the Japanese have given the name to the puzzle, its roots go
back to the Swiss Mathematician Leonhard Euler whose theory of "Latin
Squares" which he developed in 1783 shortly before his death, is
considered to be the forerunner of SuDoku.

## Rules

The rules are simple, fill a 9 by 9 cell grid such that the numbers 1 to
9 appear once only in each row, column and box. A box is a 3 by 3 grid
of which there are 9 in the main grid.

There are apparently 6 trillion possible solutions. To be uniquely
solvable a puzzle must have at least 17 initial numbers. The degree of
difficulty of a puzzle depends on the number of initial numbers given
and their distribution. As a rule of thumb, 24 initial numbers
constitute a very difficult puzzle, whereas 48 numbers constitute a
simple puzzle.

A valid Sudoku has a unique solution which can be found using an array
of solving techniques without the need to guess.

## Solving Techiques

A large number of solving techniques have been developed. Here is a non-exhaustive list grouped by difficulty level. Alternative names for solving techiques are given in brackets. Click on the link where present for more detailed information on the solving technique.

Simple Solving Techniques

- Naked Single (Singleton, lone number)
- Hidden Single

Intermediate Solving Techniques

- Pointing Pair, Pointing Triple
- Naked Pair, Naked Triple, Naked Quad, Naked Quint
- Hidden Pair, Hidden Triple, Hidden Quad

Advanced Solving Techniques

- X-Wing
- SwordFish
- JellyFish
- Squirmbag

The difficulty of a Sudoku is determined by the level of the solving techniques required to solve it.