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.
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.
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.
- Naked Single (Singleton, lone number)
The difficulty of a Sudoku is determined by the level of the solving techniques required to solve it.