Center Dot Sudoku – Medium

Center Dot Sudoku

Center Dot Sudoku is a classic 9x9 Sudoku variant with an additional region. In each of the nine 3x3 blocks, the middle cell is marked. These nine middle cells form the Center Dot Region.

Besides the normal Sudoku rules, this Center Dot Region must also contain the numbers 1 to 9 exactly once. This gives special significance to the middle cells of the blocks.

Basic Rules

• The grid consists of 9 rows and 9 columns.
• Each row contains the numbers 1 to 9 exactly once.
• Each column contains the numbers 1 to 9 exactly once.
• Each 3x3 block contains the numbers 1 to 9 exactly once.
• The nine middle cells of the 3x3 blocks form an additional Center Dot Region.
• Each number from 1 to 9 must also appear exactly once in this Center Dot Region.
• Marked Center Dot cells are normal Sudoku cells with an additional group membership.

Solving Strategies

1. Targeted Check of Middle Cells

The Center Dot cells are in fixed positions: each in the middle of a 3x3 block. Once eight of these nine cells are known, the last Center Dot cell is uniquely determined.

In the Center Dot cells, the numbers 1, 2, 4, 5, 6, 7, 8, and 9 are already present. The only missing number is 3. Therefore, the middle cell of the central 3x3 block, i.e., row 5, column 5, must be a 3.

2. Early Notation of Center Dot Candidates

Because the nine Center Dot cells are distributed across the entire grid, they can be overlooked easily. It is worthwhile to consider these cells early as their own group and note the missing numbers of the Center Dot Region separately.

3. Consider Multiple Effects of an Entry

A Center Dot cell belongs simultaneously to four areas: row, column, 3x3 block, and Center Dot Region. Each entry in such a cell therefore causes many exclusions.

In the example above, the 3 in row 5, column 5 not only removes the 3 from the row, column, and the middle block, but also from all other Center Dot cells.

Typical Solving Procedure

1. First, look for normal Sudoku entries in rows, columns, and blocks.
2. Then check the nine Center Dot cells as their own region.
3. Note which numbers are already present in this region.
4. Eliminate existing Center Dot numbers from the other Center Dot cells.
5. Immediately use Center Dot entries again for their row, column, and block.
6. Regularly verify that the Center Dot region contains no duplicate numbers.

Common Mistakes

• Guess too early instead of using the additional rule as a secure exclusion.
• Only check rows and columns and overlook the special marking of the puzzle.
• Enter a number even if it fits locally but violates the special rule.
• Fail to update candidates after each new entry.
• Misread a mark or apply it to the wrong cells.

Tips for Beginners

• Start like a normal Sudoku with well-filled rows, columns, and regions.
• Then check the special rule deliberately. Often, early secure exclusions arise there.
• Always work with the question: Which numbers are really allowed and which are excluded by a rule?
• Enter a number only when it can be clearly derived.
• If you get stuck, change your perspective: from rows to columns, from regions to the special rule, or from individual cells to candidates.

Center Dot Sudoku is a very clear extension of classic Sudoku. The additional rule is easy to understand but significantly changes the candidate situation in many cells.