Description

A Hidden Quad is four cells that are part of the same group and contain the same four candidates which are not present in any other cells in the group. The four hidden quad cells should contain more than the four candidates. This means that we can remove all other candidates from the Hidden Quad cells other than the hidden candidates.

Reasoning

If the hidden quad candidates only appear four, three or two times in the group and only in these four cells, they must be the solutions for the cells. Therefore all other candidates in the for hidden quad cells can be elliminated.

All 4 candidate do not have to appear in all 4 cells, but they must not appear in any other cells and they must not form a hidden triple.

Examples

Box Example

1, 3, 4 and 7 form the hidden quad in Box 8.

Box Example

3, 5, 6 and 9 form the hidden quad in Box 1.

Box Example

1, 2, 3 and 9 form the hidden quad in Box 6.

Algorithm

1. Fill in all the pencil marks in the puzzle.
2. For every box, row and column
1. Look for any 4 cells that have four candidates that only appear in 4 cells in that group.
2. Make sure that no 3 cells form a hidden triple.
3. Once you find a hidden quad, you can remove all the other candidates from the hidden quad cells.

Practice Puzzles

If you tap on the following links on an iOS device which has the Sudoku Tutor app installed, it will launch the app and open the practice puzzle. Tap hint once the puzzle is open to see the strategy in action.

Sample Puzzle 1
Sample Puzzle 2
Sample Puzzle 3

Next Step

Back to Sudoku Solvers or continue to next algorithm Group Bound