EXTREMEPUZZLE BOOKS

Extreme Sudoku 1, puzzle 147: 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 147 of 150 in Extreme Sudoku 1
Difficulty
Expert 6 (expert and beyond), from the final, hardest third of the book
Key technique
Nice Loops/AICs
Hard steps
16 of 40 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 (×4), ALS-XZ (×3), Cell Forcing Chains, Sue de Coq, WXYZ-Wing, X-Wing/Swordfish, 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 (16)
  1. Step 4 · WXYZ-Wing expert
    Rules out 8 from R4C4.
  2. Step 6 · Cell Forcing Chains expert
    Rules out 7 from R2C8.
  3. Step 10 · ALS-XZ expert
    Rules out 8 from R8C5.
  4. Step 11 · ALS-XZ expert
    Rules out 5 from R8C5.
  5. Step 12 · Nice Loops/AICs expert
    Rules out 6 from R6C3.
  6. Step 13 · Nice Loops/AICs expert
    Rules out 5 from R9C4.
  7. Step 15 · Sue de Coq expert
    Rules out 1 from R7C7.
  8. Step 17 · XY-Chain expert
    Rules out 5 from R6C2, R9C9.
  9. Step 20 · XY-Chain expert
    Rules out 7 from R4C1.
  10. Step 21 · Nice Loops/AICs expert
    Rules out 7 from R4C3, R5C4.
  11. Step 23 · ALS-XZ expert
    Rules out 1 from R6C3.
  12. Step 24 · Nice Loops/AICs expert
    Rules out 4 from R9C2.
  13. Step 26 · XY-Chain expert
    Rules out 8 from R5C4.
  14. Step 28 · XY-Chain expert
    Rules out 6 from R6C1.
  15. Step 36 · X-Wing/Swordfish advanced
    Rules out 1 from R4C1, R7C5.
  16. Step 37 · XY-Wing advanced
    Rules out 5 from R6C5.

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.