Asterisk Sudoku: Rules and Solving Strategy

Treat the asterisk as one more Sudoku unit

Asterisk Sudoku uses all the standard rules of a classic 9x9 puzzle: every row, every column, and every 3x3 box must contain the digits 1 through 9 exactly once. The extra rule is simple. Nine specially marked cells form an asterisk-shaped region, and those nine cells must also contain 1 through 9 once each. The marked cells are spread across the grid rather than forming one connected 3x3 box, so it helps to think of them as a fourth kind of unit. Every asterisk cell belongs to its normal row, column, and box as well as the special region. If a 6 is placed in one marked cell, 6 is immediately removed from the other eight marked cells even when they share no ordinary unit with it. Shapedoku itself uses only rows, columns, and 3x3 boxes, but the same candidate discipline applies: once an extra unit is introduced, every Note must respect all units that contain the cell.

Scan the extra region for singles and subsets

The fastest way to use the asterisk is to solve it exactly as you would solve a row or box. List which digits are already present in the nine marked cells, then look for the missing ones. A naked single appears when one marked cell has only one legal candidate after row, column, box, and asterisk restrictions are combined. A hidden single appears when a missing digit can occupy only one of the marked cells, even if that cell still shows several candidates. Pairs and triples work too. If two asterisk cells can contain only 2 and 7, those values are locked into those two positions and can be removed from the other seven marked cells. This may look unusual because the cells can be far apart on the page, but logically the deduction is no different from a naked pair in one row. The difficult part is visual tracking, not the rule itself. Mark or mentally highlight all nine asterisk positions before searching for subsets so you do not accidentally include an ordinary neighboring cell.

Use the asterisk to strengthen ordinary rows and boxes

The extra region becomes most useful when its restrictions feed back into standard Sudoku. Suppose a marked cell is one of only two places for 4 in its row, but the other row candidate lies in another marked cell. If the asterisk already forces one of those positions away from 4, the row gains a hidden single. The reverse also happens: a row placement can remove a candidate from the asterisk and create a special-region single elsewhere. Boxes often produce the strongest interactions because several marked cells may sit in different boxes while competing for the same digit. Scan in both directions. After every ordinary placement, update the asterisk. After every asterisk deduction, update the affected row, column, and box. On harder puzzles, full Notes prevent the extra unit from becoming an afterthought. If you are comfortable with Hard or Extreme Shapedoku, the useful habit is familiar: keep the candidate map clean, make one justified elimination, and immediately look for simpler consequences before hunting another advanced pattern.

1. Identify all nine marked asterisk cells before solving.
2. Treat those cells as a complete unit containing 1 through 9 once each.
3. After every placement, remove that digit from the remaining asterisk cells.
4. Search the special region for naked singles, hidden singles, pairs, and triples.
5. Use asterisk eliminations to create new placements in crossing rows, columns, and boxes.
6. Rescan the asterisk whenever an ordinary Sudoku deduction changes one of its cells.