EXTREMEPUZZLE BOOKS

Extreme Sudoku 2, puzzle 106: step-by-step solution

Step through the whole solution, or skip to the hard parts. Blue digits are placed by the walkthrough; red strike-through marks a candidate being ruled out.

Puzzle
Sudoku, number 106 of 150 in Extreme Sudoku 2
Difficulty
Expert 6 (expert and beyond), from the final, hardest third of the book
Key technique
XY-Chain
Hard steps
17 of 33 steps need an advanced or expert technique

Blue digits are placed by the walkthrough. Red strike-through marks a candidate being ruled out. Cells are named by row and column: R3C5 is row 3, column 5. Arrow keys step too.

The key technique: XY-Chain

A chain of two-candidate cells, each linked to the next. Whichever way the start cell goes, one end of the chain holds the digit, so cells seeing both ends cannot.

Other advanced and expert techniques in this solve: Nice Loops/AICs (×3), ALS-XZ (×2), Cell Forcing Chains, Empty Rectangle, Finned Fish, Sue de Coq, W-Wing, X-Wing/Swordfish, XY-Wing.

Every move above the basics is listed on the sudoku techniques page, each with a one-sentence explanation and worked examples from the books.

The hard steps, in text (17)
  1. Step 5 · X-Wing/Swordfish advanced
    Rules out 7 from R2C6, R3C6, R6C6, R9C6.
  2. Step 6 · Empty Rectangle advanced
    Rules out 7 from R2C5.
  3. Step 7 · ALS-XZ expert
    Rules out 5 from R3C9.
  4. Step 8 · Cell Forcing Chains expert
    Rules out 8 from R1C8.
  5. Step 11 · XY-Chain expert
    Rules out 5 from R3C5.
  6. Step 12 · W-Wing advanced
    Rules out 3 from R2C6, R3C6.
  7. Step 13 · Nice Loops/AICs expert
    Rules out 8 from R3C4.
  8. Step 14 · Nice Loops/AICs expert
    Rules out 8 from R6C4.
  9. Step 16 · XY-Chain expert
    Rules out 8 from R5C4.
  10. Step 18 · ALS-XZ expert
    Rules out 3 from R1C4.
  11. Step 21 · Finned Fish expert
    Rules out 3 from R8C4, R8C5.
  12. Step 22 · Sue de Coq expert
    Rules out 3 from R2C5.
  13. Step 24 · Nice Loops/AICs expert
    Rules out 8 from R2C3.
  14. Step 25 · XY-Wing advanced
    Rules out 8 from R4C2.
  15. Step 28 · XY-Chain expert
    Rules out 9 from R9C6.
  16. Step 29 · XY-Chain expert
    Rules out 3 from R2C4, R3C4.
  17. Step 31 · XY-Chain expert
    Rules out 4 from R2C3, R6C2.

The book

Volume two: 150 new classic 9×9 grids, none repeated from book 1, ramping from hard to brutal.

150 new puzzles. Expert and beyond. Puzzles by M. J. Bailey. Paperback, 5 × 8 in, 196 pages, £9.99 / $11.99. Every solution in the book carries a QR code to its own walkthrough, so you can check one step without seeing the whole answer.

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More step-by-step solutions: Extreme Sudoku 1 · Extreme Killer Sudoku 1 · Extreme Killer Sudoku 2 · Extreme Sudoku 3 · Extreme Killer Sudoku 3.