EXTREMEPUZZLE BOOKS

Extreme Sudoku 1, puzzle 79: 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 79 of 150 in Extreme Sudoku 1
Difficulty
Expert 6 (expert and beyond), from the middle third of the book
Key technique
Nice Loops/AICs
Hard steps
23 of 47 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), W-Wing (×3), Skyscraper (×2), WXYZ-Wing (×2), Cell Forcing Chains, Finned Fish, 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 (23)
  1. Step 7 · Skyscraper advanced
    Rules out 8 from R4C2, R5C9.
  2. Step 8 · W-Wing advanced
    Rules out 9 from R2C3.
  3. Step 9 · Finned Fish expert
    Rules out 3 from R8C2.
  4. Step 10 · WXYZ-Wing expert
    Rules out 1 from R5C6.
  5. Step 11 · ALS-XZ expert
    Rules out 4 from R8C7.
  6. Step 12 · Nice Loops/AICs expert
    Rules out 3 from R1C1.
  7. Step 13 · Nice Loops/AICs expert
    Rules out 3 from R2C7, R9C8.
  8. Step 17 · ALS-XZ expert
    Rules out 1 from R4C9.
  9. Step 18 · Cell Forcing Chains expert
    Rules out 8 from R3C3, R5C2; 9 from R5C1, R5C3; 1 from R5C3.
  10. Step 20 · ALS-XZ expert
    Rules out 3 from R2C3.
  11. Step 22 · Nice Loops/AICs expert
    Rules out 7 from R3C1.
  12. Step 23 · Nice Loops/AICs expert
    Rules out 7 from R3C3.
  13. Step 24 · WXYZ-Wing expert
    Rules out 1 from R2C1.
  14. Step 25 · ALS-XZ expert
    Rules out 4 from R7C8.
  15. Step 27 · Nice Loops/AICs expert
    Rules out 1 from R2C3.
  16. Step 28 · XY-Chain expert
    Rules out 5 from R6C3.
  17. Step 30 · W-Wing advanced
    Rules out 9 from R6C3.
  18. Step 32 · Skyscraper advanced
    Rules out 2 from R8C9.
  19. Step 33 · XY-Wing advanced
    Rules out 4 from R9C6.
  20. Step 34 · Nice Loops/AICs expert
    Rules out 5 from R4C4.
  21. Step 35 · Nice Loops/AICs expert
    Rules out 1 from R8C6.
  22. Step 36 · Nice Loops/AICs expert
    Rules out 1 from R2C2.
  23. Step 39 · W-Wing advanced
    Rules out 2 from R6C7, R9C9.

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.