EXTREMEPUZZLE BOOKS

Extreme Sudoku 1, puzzle 117: 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 117 of 150 in Extreme Sudoku 1
Difficulty
Expert 6 (expert and beyond), from the final, hardest third of the book
Key technique
XY-Chain
Hard steps
11 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: XY-Chain

A chain of two-candidate cells, each linked to the next. Whichever way the start cell goes, one end of the chain holds the digit, so cells seeing both ends cannot.

Other advanced and expert techniques in this solve: Nice Loops/AICs (×2), 2-String Kite, ALS-XZ, Cell Forcing Chains, Finned Fish, W-Wing, WXYZ-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 (11)
  1. Step 6 · 2-String Kite advanced
    Rules out 4 from R5C9.
  2. Step 7 · Finned Fish expert
    Rules out 8 from R4C7, R6C7.
  3. Step 8 · ALS-XZ expert
    Rules out 5 from R9C2.
  4. Step 9 · Nice Loops/AICs expert
    Rules out 6 from R4C8, R5C8.
  5. Step 12 · Nice Loops/AICs expert
    Rules out 3 from R4C9, R9C8.
  6. Step 15 · Cell Forcing Chains expert
    Rules out 3 from R1C8.
  7. Step 18 · WXYZ-Wing expert
    Rules out 5 from R1C7, R2C6.
  8. Step 23 · XY-Chain expert
    Rules out 2 from R1C3.
  9. Step 25 · W-Wing advanced
    Rules out 5 from R3C2, R7C3.
  10. Step 28 · XY-Chain expert
    Rules out 7 from R2C6.
  11. Step 29 · XY-Chain expert
    Rules out 5 from R1C6, R3C3.

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.