The Sudoku puzzle format consists of (usually) a 9 by 9 grid that must be filled with numbers. The classical Sudoku rules are as follows

1. Each row must contain all of the digits from 1 to 9, with no repetitions
2. Each column must contain all of the digits from 1 to 9, with no repetitions
3. Each sub square (3x3 grids marked by bold lines) must contain all of the digits from 1 to 9, with no repetitions

The historical inspiration of Sudoku begins with the Latin square, a type of grid problem similar to Sudoku in that it consists of an N by N grid where N digits (or letters) must be arranged in accordance with rules 1 and 2 of Sudoku (the sub square had yet to be invented). The first known example of a 9 by 9 Latin Square is from a monograph over 300 years ago, written by the Korean mathematician Choi Seok-Jeong. However, the Latin Square was popularized in the west by the legendary mathematician Leonhard Euler, who independently invented it almost 70 years after Choi Seok-Jeong.

According to Christian Boyer (idk who this is), on July 6th, 1895, the french newspaper La France published the earliest known example of a "true" Sudoku puzzle following all 3 rules of modern Sudokus, although french newspapers had previously been publishing similar (but different) puzzles prior. The name "Sudoku" itself (meaning "single digit") comes from Japanese magazines in which the game was popular during the 80s and 90s.

In Modern times, the Sudoku puzzle scene has been blessed with the addition of numerous variants, that add on additional rules, elevating the puzzle. The puzzle in the provided image is one such example, known as the "miracle Sudoku". It comes with 3 additional rules on top of the 3 original.

4. If 2 digits are separated by a knight's move in chess, then they cannot be the same
5. If 2 digits are separated by a king's move in chess, then they cannot be the same
6. If 2 digits are adjacent (left, right, down or top, none of the diagonal directions), then they cannot be consecutive (ex - 5 and 6, or 7 and 8)

As can be seen from the fact that only 2 digits have been provided to start with, modern Sudoku authors are big fans of making puzzles that appear to be impossible to solve. This of course, is my favorite part about modern Sudoku. If you want to try to solve the miracle Sudoku, here is the link.

Some of the other example variants add new rules, such as German whispers, Dutch whispers, Renban lines, Entropy lines, Thermometer lines, killer cages and many more. You can find a massive collection of these puzzles on the logic-masters website.

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