EXTREMEPUZZLE BOOKS

Extreme Sudoku 2, puzzle 121: 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 121 of 150 in Extreme Sudoku 2
Difficulty
Expert 7 (expert and beyond), from the final, hardest third of the book
Key technique
Nice Loops/AICs
Hard steps
25 of 48 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: Nice Loops/AICs

An alternating chain of strong and weak links. Whichever end is true, the conclusion holds, so the digit at the marked cell is ruled out.

Other advanced and expert techniques in this solve: ALS-XZ (×7), Finned Fish (×2), Cell Forcing Chains, Empty Rectangle, Skyscraper, Sue de Coq, Unique Rectangles, W-Wing, WXYZ-Wing, XY-Chain.

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 (24 of 25)
  1. Step 3 · Skyscraper advanced
    Rules out 9 from R1C5, R5C4, R6C4.
  2. Step 9 · WXYZ-Wing expert
    Rules out 4 from R1C3.
  3. Step 10 · ALS-XZ expert
    Rules out 4 from R4C5.
  4. Step 11 · Empty Rectangle advanced
    Rules out 4 from R2C6.
  5. Step 12 · ALS-XZ expert
    Rules out 6 from R6C4.
  6. Step 13 · ALS-XZ expert
    Rules out 2 from R6C4.
  7. Step 15 · Nice Loops/AICs expert
    Rules out 6 from R4C1.
  8. Step 16 · Nice Loops/AICs expert
    Rules out 4 from R7C6.
  9. Step 17 · Nice Loops/AICs expert
    Rules out 4 from R2C5.
  10. Step 18 · Cell Forcing Chains expert
    Rules out 7 from R1C9.
  11. Step 21 · W-Wing advanced
    Rules out 5 from R1C6.
  12. Step 22 · Finned Fish expert
    Rules out 6 from R8C3.
  13. Step 23 · Sue de Coq expert
    Rules out 5 from R5C4, R6C5.
  14. Step 25 · Nice Loops/AICs expert
    Rules out 6 from R9C2.
  15. Step 26 · Nice Loops/AICs expert
    Rules out 6 from R8C4.
  16. Step 27 · Nice Loops/AICs expert
    Rules out 6 from R5C2.
  17. Step 31 · ALS-XZ expert
    Rules out 4 from R7C1.
  18. Step 32 · ALS-XZ expert
    Rules out 3 from R9C3.
  19. Step 33 · ALS-XZ expert
    Rules out 2 from R9C1.
  20. Step 34 · ALS-XZ expert
    Rules out 5 from R8C4.
  21. Step 35 · Nice Loops/AICs expert
    Rules out 5 from R2C6.
  22. Step 36 · Nice Loops/AICs expert
    Rules out 3 from R2C4, R9C6.
  23. Step 39 · Finned Fish expert
    Rules out 5 from R8C3.
  24. Step 42 · Unique Rectangles advanced
    Rules out 5 from R7C1; 8 from R7C1.

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.