At the @nrich (enrich) webpage of maths.org you can find more than thirty different kind of Sudoku’s. A lot of them are designed by Henry Kwok. One of them is called Pole Star Sudoku (also known as Duplex Difference Sudoku).
The Pole Star Sudoku is a variation of Sudoku on a standard 9×9 grid which contains a set of special clue-numbers. These are small numbers placed always on the border lines between selected pairs of neighboring cells of the grid. Each clue-number is the difference between the two numbers that should be in the neighboring cells just to the left and to the right from that clue-number.
For example, a clue-number 7 on the border line means that one of the possible answers in the cell on the left can be 9, 2, 8 or 1. If we choose 9 for the answer in the cell, it means that the answer in the cell next to it on the right is 2. If we choose the answer 2, it means that the answer on the right is 9.
In addition, there are the symbols 1> and 1< on some of the border lines. The 1> means the answer in the cell on the left is 1 greater in value than the answer in the cell on the right. Similarly, the 1< would indicate that the answer in the cell on the left is 1 less than the answer in the cell on the right.
The remaining rules are as in a standard Sudoku: the object of the puzzle is to fill in the whole 9×9 grid with numbers 1 through 9 (one number per cell) so that each horizontal line, each vertical line, and each of the nine 3×3 squares (outlined with the bold lines) must contain all the nine different numbers 1 through 9.
The maths.org website is a great source of Sudoku variants and for people who want to try different kind of Sudoku’s. You can also find Sudoku’s in easy printable format including the solution. Happy Puzzling!