International Sudoku Day

What the day is

International Sudoku Day is observed on September 9, the date written as 9/9, which suits a puzzle built on nine rows of nine.

Origin and background

The puzzle is not Japanese, though the name is. The modern form was devised by a retired American architect and puzzle constructor, and first published in a United States puzzle magazine in 1979 under the name Number Place. A Japanese publisher picked it up in the mid-1980s, gave it the abbreviated name it now carries, and refined the conventions, particularly the rule that the givens should be symmetrically arranged. It returned to the English-speaking world as an apparently exotic import in 2004, when a New Zealander who had written software to generate the puzzles persuaded a London newspaper to print them, and the boom followed within months.

Its mathematical ancestry runs further back. A sudoku grid is a Latin square with an extra constraint, and Latin squares were studied formally in the eighteenth century by Euler, long before anyone thought to make a puzzle from one.

The most satisfying result about sudoku is recent. Players had long noticed that puzzles with very few givens tended to have more than one solution, and the open question was how few clues a puzzle could carry and still have exactly one answer. Many valid 17-clue puzzles were known, and none with 16 had ever been found. In 2012 a team settled it by exhaustive computer search, checking every possible arrangement in a way that would have been impossible a decade earlier, and proved that 16 clues can never be enough. Seventeen is the minimum.

Solving needs no arithmetic at all. The digits are labels, and the puzzle would work identically with nine colors or nine symbols.

How to take part

• Do one in pen and commit to the logic instead of guessing.
• Learn one technique beyond scanning, such as looking for pairs in a row.
• Try a variant grid if the standard one has stopped being interesting.
• Remember it involves no arithmetic. The numbers are only labels.