[Open.ogc] No, sudoku does not come from Japan

[Vatas Korenev]

It was under the name “Number Place”, the place of numbers, that the game
was born in 1979. Created by Howard Garns (1905-1989), American architect,
this grid of 81 squares (9 by 9) was first published in the small New York
magazine Dell Pencil Puzzles and Word Games. Without making much noise at
the time. Its founder even died before the grid enjoyed worldwide success
from 2004. That year, The Times magazine proposed his invention for the
first time, thus propagating its concept under the name of… sudoku.

>From the magic square to the single number

“Sudoku” is the diminutive of “Suji wa dokushin ni kagiru” which means in
Japanese “the number must remain unique”. The game became popular in the
1980s, thanks to Maki Kaji and her publishing house Nikoli. The “godfather
of Sudoku” prints these little grids in the first issues of his games
magazine. He adds an additional constraint: only 30 numbers out of the 81
squares of the grid are written on the grid. It is thanks to sudoku that
the publication has enjoyed great success since its beginnings, and has
more than 50,000 subscribers.

To find the ancestors of sudoku, we must look at China and Switzerland with
the “magic square” and the “Latin square”. The magic square has its origins
in China in - 650 BC. It then comprised a square of three by three called
Luoshu, named after the Luo River. In this table, the sum of each row must
be equal to the sum of each column, and that of each diagonal. For example,
if adding the numbers in a row gives 10, then the numbers on each diagonal
and each column must also equal 10.

Brain teaser

The magic square is itself the parent of the Latin square. This problem is
said to have been invented by Leonhard Euler, an 18th century Swiss
mathematician. Here, each row and each column must be completed with a
number going from 0 to n-1, n being the number of boxes per line, per
column or even per diagonal. For example, in a 3 by 3 square, the numbers
will go from 0 to 2. Like a mini sudoku, then.

If you were too lazy to read the article, here is how the history of the
most famous game grid could be summarized: sudoku is an American game -
https://sudokuhit.com/medium/ , whose name is borrowed from Japanese, and
which is made up of a mosaic of Latin squares, but coming from Switzerland,
themselves derived from a Chinese problem. Quite simple, right?

What does the difficulty of a sudoku depend on?

Almost exclusively from the number of clues they give you, the numbers that
are already located. If the amount of initial data is less than 20, for
example, the difficulty increases. And it can go up a lot depending on
where those numbers are located. As a curious fact, with less than 17 clues
a sudoku with a single solution is no longer possible. It is a difficult
point to define, because it also varies with the habituation of each
player. Each publisher usually determines its own criteria, although there
are some that are more or less universal. It can be measured, for example,
from the methods necessary to solve it. If I can solve a sudoku puzzle with
the first method, sequentially, it is too simple. If another method is also
needed, such as “invisible pairs”, Sudoku is medium. And so we increase the
difficulty.
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