EXTREMEPUZZLE BOOKS

Extreme Sudoku 3, puzzle 42: 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 42 of 150 in Extreme Sudoku 3
Difficulty
Expert 5 (expert and beyond), from the opening third of the book
Key technique
Nice Loops/AICs
Hard steps
20 of 45 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: 2-String Kite (×3), W-Wing (×3), ALS-XZ (×2), Skyscraper (×2), Cell Forcing Chains, Finned Fish, WXYZ-Wing, 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 (20)
  1. Step 3 · 2-String Kite advanced
    Rules out 1 from R3C9.
  2. Step 4 · 2-String Kite advanced
    Rules out 3 from R3C3.
  3. Step 5 · W-Wing advanced
    Rules out 4 from R8C1, R8C3, R9C8.
  4. Step 8 · Finned Fish expert
    Rules out 6 from R4C6, R6C6.
  5. Step 9 · Nice Loops/AICs expert
    Rules out 9 from R6C1.
  6. Step 10 · WXYZ-Wing expert
    Rules out 6 from R5C3.
  7. Step 15 · Skyscraper advanced
    Rules out 3 from R2C8.
  8. Step 16 · ALS-XZ expert
    Rules out 1 from R7C3.
  9. Step 17 · XY-Wing advanced
    Rules out 9 from R8C3.
  10. Step 21 · Nice Loops/AICs expert
    Rules out 7 from R1C6.
  11. Step 22 · Nice Loops/AICs expert
    Rules out 6 from R1C6.
  12. Step 24 · Nice Loops/AICs expert
    Rules out 8 from R9C4.
  13. Step 25 · Cell Forcing Chains expert
    Rules out 9 from R1C2.
  14. Step 28 · W-Wing advanced
    Rules out 6 from R3C7, R4C9, R6C9.
  15. Step 30 · ALS-XZ expert
    Rules out 9 from R2C9, R4C8.
  16. Step 31 · Nice Loops/AICs expert
    Rules out 8 from R9C5.
  17. Step 32 · Nice Loops/AICs expert
    Rules out 8 from R5C4.
  18. Step 33 · 2-String Kite advanced
    Rules out 8 from R9C6.
  19. Step 34 · Skyscraper advanced
    Rules out 8 from R4C8, R8C9.
  20. Step 36 · W-Wing advanced
    Rules out 1 from R9C6.

The book

Volume three: 150 new classic 9×9 grids, none repeated from books 1 or 2, 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.

On Amazon from 8 Dec About the book

Try a free sample puzzle (PDF) · prints on A4 or US Letter

More step-by-step solutions: Extreme Sudoku 1 · Extreme Killer Sudoku 1 · Extreme Sudoku 2 · Extreme Killer Sudoku 2 · Extreme Killer Sudoku 3.