EXTREMEPUZZLE BOOKS

Extreme Sudoku 2, puzzle 91: 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 91 of 150 in Extreme Sudoku 2
Difficulty
Expert 5 (expert and beyond), from the middle third of the book
Key technique
Nice Loops/AICs
Hard steps
13 of 33 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 (×2), XY-Chain (×2), Cell Forcing Chains, Finned Fish, W-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 (13)
  1. Step 9 · W-Wing advanced
    Rules out 4 from R4C6.
  2. Step 10 · ALS-XZ expert
    Rules out 3 from R8C5.
  3. Step 11 · Nice Loops/AICs expert
    Rules out 3 from R7C3.
  4. Step 13 · Cell Forcing Chains expert
    Rules out 2 from R4C3, R5C6, R9C2; 4 from R5C2.
  5. Step 16 · ALS-XZ expert
    Rules out 2 from R3C4.
  6. Step 17 · Nice Loops/AICs expert
    Rules out 2 from R4C4.
  7. Step 19 · Nice Loops/AICs expert
    Rules out 7 from R1C4.
  8. Step 20 · Nice Loops/AICs expert
    Rules out 5 from R7C5.
  9. Step 22 · Finned Fish expert
    Rules out 7 from R3C4.
  10. Step 23 · Nice Loops/AICs expert
    Rules out 7 from R4C4.
  11. Step 27 · X-Wing/Swordfish advanced
    Rules out 7 from R7C4, R7C5, R9C1, R9C3 and 1 more.
  12. Step 28 · XY-Chain expert
    Rules out 7 from R4C1.
  13. Step 30 · XY-Chain expert
    Rules out 7 from R8C1.

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.