QED Logic quod erat demonstrandum
Logic Ascent

How to solve this 8×8 logic puzzle

8×8 · steady · 16 steps

A
B
C
D
E
F
G
H
1
2
3
4
5
6
7
8
1
2
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3
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4
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5
6
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7
8
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This 8×8 board rates as steady (difficulty score 804). Solving it from scratch takes 26 logical steps, using 6 techniques: Single cell, Row/column exclusion, Adjacency clear, Region confinement, Line confinement, Shape squeeze. Every step below is forced: nothing here is a guess.

  1. First, every open cell left in this region sits in row 1, so its marker has to land there; that clears every other open cell in the row.
  2. Next, every open cell left in this region sits in column A, so its marker has to land there; that clears every other open cell in the column.
  3. Then, every open cell left in this region sits in column B, so its marker has to land there; that clears every other open cell in the column.
  4. After that, every open cell left in this region sits in column C, so its marker has to land there; that clears every other open cell in the column.
  5. Now, every open cell left in this region sits in column D, so its marker has to land there; that clears every other open cell in the column.
  6. From there, every open cell left in this region sits in column E, so its marker has to land there; that clears every other open cell in the column.
  7. Following that, place a marker at F1. Region has exactly one open cell left. Placing here clears the rest of row 1, column F, and its region. Placing here also clears its 2 touching neighbours.
  8. First, every open cell in row 8 belongs to the same region, so that region's marker has to be somewhere in this row; that clears every other open cell in the region.
  9. Next, place a marker at A7. Row 7 has exactly one open cell left. Placing here clears the rest of row 7, column A, and its region.
  10. Then, every remaining candidate in this region touches D5, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
  11. After that, place a marker at D6. Region has exactly one open cell left. Placing here clears the rest of row 6, column D, and its region. Placing here also clears its 2 touching neighbours.
  12. Now, place a marker at C4. Region has exactly one open cell left. Placing here clears the rest of row 4, column C, and its region. Placing here also clears its 1 touching neighbour.
  13. From there, place a marker at B2. Region has exactly one open cell left. Placing here clears the rest of row 2, column B, and its region.
  14. Following that, place a marker at E3. Region has exactly one open cell left. Placing here clears the rest of row 3, column E, and its region.
  15. First, place a marker at H5. Region has exactly one open cell left. Placing here clears the rest of row 5, column H, and its region.
  16. Finally, place a marker at G8. Region has exactly one open cell left.

The key move here was Shape squeeze. Once you can spot that, this board falls quickly. ∎