EXTREMEPUZZLE BOOKS

Extreme Sudoku 1, puzzle 4: 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 4 of 150 in Extreme Sudoku 1
Difficulty
Expert 5 (expert and beyond), from the opening third of the book
Key technique
Nice Loops/AICs
Hard steps
18 of 38 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 (×4), Finned Fish (×2), W-Wing (×2), XY-Chain (×2), Skyscraper, WXYZ-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 (18)
  1. Step 4 · ALS-XZ expert
    Rules out 5 from R5C4.
  2. Step 5 · Nice Loops/AICs expert
    Rules out 8 from R6C1.
  3. Step 6 · Nice Loops/AICs expert
    Rules out 3 from R9C9.
  4. Step 9 · Finned Fish expert
    Rules out 6 from R7C8, R9C8.
  5. Step 10 · Nice Loops/AICs expert
    Rules out 2 from R3C8.
  6. Step 12 · WXYZ-Wing expert
    Rules out 6 from R8C8.
  7. Step 13 · Finned Fish expert
    Rules out 6 from R3C2.
  8. Step 18 · Skyscraper advanced
    Rules out 8 from R2C1, R6C2.
  9. Step 20 · ALS-XZ expert
    Rules out 7 from R9C5.
  10. Step 21 · Nice Loops/AICs expert
    Rules out 5 from R9C5.
  11. Step 22 · Nice Loops/AICs expert
    Rules out 5 from R8C5.
  12. Step 23 · ALS-XZ expert
    Rules out 3 from R2C6.
  13. Step 24 · ALS-XZ expert
    Rules out 1 from R7C6.
  14. Step 25 · Nice Loops/AICs expert
    Rules out 7 from R8C5.
  15. Step 28 · XY-Chain expert
    Rules out 7 from R8C7.
  16. Step 31 · W-Wing advanced
    Rules out 8 from R1C8, R4C9.
  17. Step 33 · W-Wing advanced
    Rules out 7 from R9C9.
  18. Step 34 · XY-Chain expert
    Rules out 5 from R9C4.

The book

Classic 9×9 sudoku with the warm-up taken out. Every grid sits at the top of the expert range or beyond it, and the book ramps from hard to brutal.

150 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 Killer Sudoku 1 · Extreme Sudoku 2 · Extreme Killer Sudoku 2 · Extreme Sudoku 3 · Extreme Killer Sudoku 3.