By David Willey
BBC News, Rome
Puzzle addicts from more than 20 countries are gathering in the Italian city of Lucca to compete in the first World Sudoku Championships.
Puzzlers are set a range of Sudoku challenges at the tournament
Sudoku, a pencil and paper number game of logic, originated in Japan.
After a preliminary qualifying round, about 100 expert puzzle-solvers from around the world are taking part in the tournament.
Presiding over the competition is Wayne Gould, who has helped popularise the numbers puzzle.
After retiring from work as a criminal court judge in Hong Kong, Mr Gould, who comes from New Zealand, started a new career as puzzle master.
He developed a computer program that generates an almost infinite number of Sudoku puzzles.
He offered them free for publication in a London newspaper and readers became hooked almost immediately.
"It became a craze in England within two weeks of its appearing in The Times," he said.
His puzzles now appear in more than 400 newspapers around the world.
His Sudoku computer programme took him seven years to perfect.
There is a simplified version which substitutes shapes and colours for numbers, which children as young as three can be taught to play.
HOW TO PLAY SUDOKU
Grid: Classic Sudoku is a grid divided into nine 3x3 boxes
Aim: Fill grid so that every row, column and 3x3 box contains the digits from 1 to 9, without repeating
Skills: Reasoning and logic. No arithmetic needed
At the tournament in Lucca, competitors will tackle the Classic 9x9 Sudoku grids as well as a number of original variations including Diagonal Sudoku, Irregular Sudoku, Sum Sudoku, Toroidal Sudoku, Odd/Even Sudoku.
The traditional newspaper crossword puzzle, after more than 100 years of existence, is facing stiff competition from the Sudoku craze.
And if this weekend's World Championship attracts sufficient interest, it could become a regular event.