EXTREMEPUZZLE BOOKS

Extreme Sudoku 2, puzzle 25: 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 25 of 150 in Extreme Sudoku 2
Difficulty
Expert 6 (expert and beyond), from the opening third of the book
Key technique
Nice Loops/AICs
Hard steps
25 of 50 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 (×6), Finned Fish (×3), 2-String Kite, Cell Forcing Chains, Empty Rectangle, Skyscraper, W-Wing, WXYZ-Wing, XY-Chain, 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 (24 of 25)
  1. Step 4 · Skyscraper advanced
    Rules out 1 from R1C4, R2C1, R2C3.
  2. Step 8 · Empty Rectangle advanced
    Rules out 1 from R4C8.
  3. Step 9 · Finned Fish expert
    Rules out 1 from R7C1, R9C1.
  4. Step 10 · Finned Fish expert
    Rules out 1 from R5C8.
  5. Step 11 · Nice Loops/AICs expert
    Rules out 1 from R1C8.
  6. Step 12 · Nice Loops/AICs expert
    Rules out 4 from R2C3.
  7. Step 14 · ALS-XZ expert
    Rules out 4 from R6C9.
  8. Step 15 · Nice Loops/AICs expert
    Rules out 9 from R5C8.
  9. Step 17 · ALS-XZ expert
    Rules out 4 from R4C7.
  10. Step 18 · Nice Loops/AICs expert
    Rules out 5 from R6C3.
  11. Step 19 · Nice Loops/AICs expert
    Rules out 4 from R6C7.
  12. Step 21 · Finned Fish expert
    Rules out 2 from R4C8.
  13. Step 25 · 2-String Kite advanced
    Rules out 8 from R6C7.
  14. Step 26 · WXYZ-Wing expert
    Rules out 8 from R6C1.
  15. Step 27 · Nice Loops/AICs expert
    Rules out 8 from R6C9.
  16. Step 28 · Cell Forcing Chains expert
    Rules out 8 from R1C2.
  17. Step 32 · W-Wing advanced
    Rules out 8 from R7C9.
  18. Step 33 · ALS-XZ expert
    Rules out 8 from R7C4.
  19. Step 34 · ALS-XZ expert
    Rules out 6 from R9C4.
  20. Step 35 · ALS-XZ expert
    Rules out 2 from R7C9.
  21. Step 36 · Nice Loops/AICs expert
    Rules out 8 from R5C9.
  22. Step 37 · ALS-XZ expert
    Rules out 4 from R4C8.
  23. Step 39 · Nice Loops/AICs expert
    Rules out 2 from R7C8.
  24. Step 42 · XY-Wing advanced
    Rules out 8 from R5C8.

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.