QED Logic quod erat demonstrandum
Logic Ascent

How to solve this 8×8 logic puzzle

8×8 · steady · 14 steps

A
B
C
D
E
F
G
H
1
2
3
4
5
6
7
8
1
2
3
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4🐱
5
6
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7
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8
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This 8×8 board rates as steady (difficulty score 811). Solving it from scratch takes 25 logical steps, using 6 techniques: Single cell, Row/column exclusion, Adjacency clear, Region confinement, Paired regions, Shape squeeze. Every step below is forced: nothing here is a guess.

  1. First, every open cell left in this region sits in row 3, so its marker has to land there; that clears every other open cell in the row.
  2. Next, together, two regions have open cells only in rows 1 and 2; with one marker each, those two markers have to fill exactly those two rows, so every other region's open cells there can be cleared.
  3. Then, every open cell left in this region sits in row 4, so its marker has to land there; that clears every other open cell in the row.
  4. After that, every remaining candidate in this region touches A2 and B2, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
  5. Now, every remaining candidate in this region touches C5 and D5, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
  6. From there, together, two regions have open cells only in columns A and B; with one marker each, those two markers have to fill exactly those two columns, so every other region's open cells there can be cleared.
  7. Following that, place a marker at C2. Region has exactly one open cell left. Placing here clears the rest of row 2, column C, and its region. Placing here also clears its 1 touching neighbour.
  8. First, place a marker at A3. Region has exactly one open cell left. Placing here clears the rest of row 3, column A, and its region.
  9. Next, place a marker at D4. Region has exactly one open cell left. Placing here clears the rest of row 4, column D, and its region. Placing here also clears its 1 touching neighbour.
  10. Then, place a marker at E6. Region has exactly one open cell left. Placing here clears the rest of row 6, column E, and its region. Placing here also clears its 2 touching neighbours.
  11. After that, place a marker at F8. Region has exactly one open cell left. Placing here clears the rest of row 8, column F, and its region. Placing here also clears its 1 touching neighbour.
  12. Now, place a marker at B5. Region has exactly one open cell left. Placing here clears the rest of row 5, column B, and its region.
  13. From there, place a marker at H7. Region has exactly one open cell left. Placing here clears the rest of row 7, column H, and its region.
  14. Finally, place a marker at G1. Region has exactly one open cell left.

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