EXTREMEPUZZLE BOOKS

Extreme Sudoku 2, puzzle 27: 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 27 of 150 in Extreme Sudoku 2
Difficulty
Expert 7 (expert and beyond), from the opening third of the book
Key technique
Nice Loops/AICs
Hard steps
23 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: XY-Chain (×3), XY-Wing (×3), ALS-XZ (×2), Coloring/Multi-coloring (×2), Cell Forcing Chains, Finned Fish, Skyscraper, WXYZ-Wing, X-Wing/Swordfish.

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 3 · ALS-XZ expert
    Rules out 4 from R6C4.
  2. Step 4 · ALS-XZ expert
    Rules out 2 from R9C9.
  3. Step 5 · Nice Loops/AICs expert
    Rules out 9 from R3C5, R3C6.
  4. Step 8 · Finned Fish expert
    Rules out 5 from R7C2, R8C3.
  5. Step 9 · Nice Loops/AICs expert
    Rules out 9 from R2C5.
  6. Step 12 · Nice Loops/AICs expert
    Rules out 9 from R6C8.
  7. Step 13 · Nice Loops/AICs expert
    Rules out 9 from R6C3.
  8. Step 14 · Nice Loops/AICs expert
    Rules out 4 from R1C3.
  9. Step 15 · Nice Loops/AICs expert
    Rules out 9 from R4C8.
  10. Step 16 · Nice Loops/AICs expert
    Rules out 3 from R8C3; 8 from R8C3.
  11. Step 17 · Nice Loops/AICs expert
    Rules out 5 from R1C2.
  12. Step 18 · Cell Forcing Chains expert
    Rules out 9 from R1C9.
  13. Step 21 · X-Wing/Swordfish advanced
    Rules out 5 from R1C3, R1C8.
  14. Step 22 · XY-Chain expert
    Rules out 3 from R1C2.
  15. Step 23 · XY-Chain expert
    Rules out 2 from R9C3.
  16. Step 24 · XY-Chain expert
    Rules out 4 from R9C3.
  17. Step 25 · Coloring/Multi-coloring expert
    Rules out 4 from R1C8.
  18. Step 27 · Coloring/Multi-coloring expert
    Rules out 4 from R6C8.
  19. Step 29 · WXYZ-Wing expert
    Rules out 2 from R9C5, R9C6.
  20. Step 33 · XY-Wing advanced
    Rules out 5 from R8C4.
  21. Step 35 · XY-Wing advanced
    Rules out 8 from R7C7.
  22. Step 39 · Skyscraper advanced
    Rules out 8 from R4C9, R6C6.
  23. Step 40 · XY-Wing advanced
    Rules out 8 from R5C9.

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.