A sudoku is a rectangular matrix of nine by nine cells. In eacht cell belongs a figure. These 81 cells are ordered in 3 * 3 ninesomes. All nine ninesomes are rectangular and contain 3 * 3 cells each.

Put the figures from 1 to 9 in the cells in such a way that no double figures occur, nor in any row of nine cells, nor in any column of nine cells, nor in any ninesome.

Some cells already contain a figure. The more figures a sudoku provides, the easier the problem, because you have to place the remaining figures on the basis of the figures given. In principle, only one solution is possible. 

General approach

Most sudoku's can be solved by applying the following strategy:

1 - Check for sudoku's by using the spotlight method.

2 - Fill in the duo's and use especially pairs of duo's as lever.

3 - Execute step 1 again.

4 - Look for special patterns an use them.

5 - Execute step 1 again.

6 - Start the administration at the most promising spots, preferably with a maximum of three candidates per cel, and look for patterns in the administration.

7 - Execute step 1 again.