Sudoku is everyone's favorite, almost a mathematical puzzle. The playing field is a square with the size of 9 by 9 cells. It is divided into smaller squares of 3 cells, which means that the whole field - 81 cells.

According to the conditions, at the beginning of the game, some squares are already filled with numbers. And it is on the basis of the location of these numbers we will solve the puzzle. The level of filling the square is proportional to the ease of the game.

**The only goal of the game is to fill all the squares with numbers so that in each column, row and 3-by-3 square each number is not repeated.**

The simplest subspecies of Sudoku is the so-called "odd-even". Under the terms of this game, in addition to the original numbers, there is another clue - some cells are filled with a certain color, which, depending on the rules, means - there is an even or odd number. But this modification, because of its triviality, is not particularly widespread.

There are other subspecies with simple complications - the square instead of nine by nine cells has the size of fifteen by fifteen, sixteen by sixteen, and so on.

There is another subspecies, several orders of magnitude higher in complexity. In it, instead of tips in the form of filled cells with numbers, only the sum of the numbers in separate groups is specified - that is, the site is again divided into rectangular blocks of different sizes, and somewhere near the sum of the numbers of the block is indicated.

Another variation of Sudoku, where the blocks are not square, but arbitrary shape with the same number of cells.

There are also simple sudoku for children - 4 by 4, 2 by 2. But for the adult generation, they are of little interest. Another curious modification is diagonal sudoku. Here, the field is not divided into smaller squares, as in conventional Sudoku, and on the diagonal. They need each row, column and the maximum two diagonals to place numbers so that they would meet once.

The real popularity of Sudoku gained only in modern times, when a certain Japanese magazine began to regularly print on the back of the puzzle. Maybe the game would have remained popular only in the countries of the Rising Sun, but here, in 2004, the British "Times" also began to print this game, and spread it across Europe. The fever has gathered momentum and continues to this day. Even some regional magazines already consider Sudoku as an integral component of their material.

Sudoku lovers, over time, develop a certain algorithm by which they solve puzzles. Here are some of the most common tips to easily solve Sudoku.

- Writing down possible numbers in small handwriting in the corner of a cell. You will then be able to see the numbers that should occupy that cell on the "big picture".
- Playing Sudoku quickly is not a good idea. It is necessary to lay out the acquired information in your brain, to remember, to think, to make assumptions... This game is actually considered relaxing. Depending on difficulty, some puzzles can be solved almost instantly, others can take days.
- Be attentive. Check your actions often. A single mistake at the beginning of the puzzle can lead to the inability to solve the game. Many people say that if you can not solve Sudoku - then you should try to solve it later. The solution may come to you during the most normal, everyday activities.
- Many people start solving Sudoku in such an order as if they were reading it - left to right, left to right - and so down the squares... This may well be called a false tactic. The first thing to do is to take the squares that are the least filled. When you fill it - it becomes an invaluable clue in solving the others. When you fill a square, be sure to check the column, row, and three-by-three block in which the square you are filling is located. Make sure that the other 8 digits are not duplicated!
- If there is one empty cell left in the column or row, fill it in. Just look through the column/row/square and see what number you haven't written here yet.
- When there are several empty cells in the playing field, and no matter what, only one digit fits for a certain number, then that is what you should write in the cell. Before writing it down, check to see if the number you are writing down occurs in other cells by column, row, and square.

Bertham Felgenhauer counted the number of theoretical combinations in a standard 9-by-9 field, and concluded that **it is possible to make 6.670.903.752.021.072.936.960 different Sudoku.**