EXTREMEPUZZLE BOOKS

Extreme Sudoku 2, puzzle 15: 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 15 of 150 in Extreme Sudoku 2
Difficulty
Expert 5 (expert and beyond), from the opening third of the book
Key technique
Nice Loops/AICs
Hard steps
22 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 (×5), XY-Chain (×3), 2-String Kite, Finned Fish, Skyscraper, W-Wing, WXYZ-Wing, XY-Wing, XYZ-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 (22)
  1. Step 4 · WXYZ-Wing expert
    Rules out 7 from R8C7.
  2. Step 5 · ALS-XZ expert
    Rules out 8 from R4C7, R5C6.
  3. Step 12 · 2-String Kite advanced
    Rules out 2 from R2C2.
  4. Step 13 · Finned Fish expert
    Rules out 2 from R3C2.
  5. Step 14 · ALS-XZ expert
    Rules out 5 from R3C2.
  6. Step 15 · ALS-XZ expert
    Rules out 5 from R4C2, R5C2.
  7. Step 16 · ALS-XZ expert
    Rules out 4 from R1C3, R4C2.
  8. Step 18 · XY-Chain expert
    Rules out 7 from R8C3.
  9. Step 19 · ALS-XZ expert
    Rules out 7 from R1C2, R7C3.
  10. Step 21 · Nice Loops/AICs expert
    Rules out 2 from R1C5.
  11. Step 23 · Nice Loops/AICs expert
    Rules out 6 from R1C2.
  12. Step 24 · Nice Loops/AICs expert
    Rules out 6 from R7C5.
  13. Step 25 · Nice Loops/AICs expert
    Rules out 6 from R2C4.
  14. Step 26 · Nice Loops/AICs expert
    Rules out 6 from R2C6.
  15. Step 32 · W-Wing advanced
    Rules out 3 from R7C2.
  16. Step 33 · Nice Loops/AICs expert
    Rules out 4 from R3C5.
  17. Step 34 · Nice Loops/AICs expert
    Rules out 6 from R6C1.
  18. Step 36 · Skyscraper advanced
    Rules out 3 from R7C1, R8C4.
  19. Step 38 · XYZ-Wing advanced
    Rules out 6 from R7C2, R8C2.
  20. Step 39 · XY-Chain expert
    Rules out 3 from R5C2.
  21. Step 40 · XY-Chain expert
    Rules out 7 from R7C7, R7C9, R8C2.
  22. Step 44 · XY-Wing advanced
    Rules out 9 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.