MXQ Adv. Sudoku

4-Cell Chains

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In the example below, a 4-cell chain occurs in row A. A 1, 2, 8 or 9 occur in 4 cells meaning that those digits can only occur in those row-cells: A3, A4, A6 & A8. 1, 2, 8 or 9 can be removed from all other cells in row A: A1, A2, A5, A7 & A9. 4-cell chains are the last pattern in the solution process before the solution cascades. Simpler grids cascade almost immediately. Extremely difficult grids cascade only after about 80 steps.

4 Cell Chains