QED Logic quod erat demonstrandum
Logic Ascent

How to solve this 8×8 logic puzzle

8×8 · steady · 20 steps

A
B
C
D
E
F
G
H
1
2
3
4
5
6
7
8
1
2
🐱
3
4🐱
🐱
5
🐱
6
🐱
7
🐱
🐱
8
🐱

This 8×8 board rates as steady (difficulty score 902). Solving it from scratch takes 30 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 6, 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 row 5, so its marker has to land there; that clears every other open cell in the row.
  3. Then, every open cell left in this region sits in column G, 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 H, so its marker has to land there; that clears every other open cell in the column.
  5. Now, 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.
  6. From there, every open cell in row 7 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.
  7. Following that, every remaining candidate in this region touches E3, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
  8. First, every remaining candidate in this region touches F3 and H3 and F4 and H4, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
  9. Next, every remaining candidate in this region touches C5, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
  10. Then, every remaining candidate in this region touches E4, 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 F2. Region has exactly one open cell left. Placing here clears the rest of row 2, column F, and its region. Placing here also clears its 2 touching neighbours.
  12. Now, place a marker at H1. Region has exactly one open cell left. Placing here clears the rest of row 1, column H, and its region.
  13. From there, place a marker at G4. Region has exactly one open cell left. Placing here clears the rest of row 4, column G, and its region.
  14. Following that, every remaining candidate in this region touches D6, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
  15. First, every remaining candidate in this region touches B7, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
  16. Next, place a marker at A7. Region has exactly one open cell left. Placing here clears the rest of row 7, column A, and its region. Placing here also clears its 2 touching neighbours.
  17. Then, place a marker at C6. Region has exactly one open cell left. Placing here clears the rest of row 6, column C, and its region. Placing here also clears its 1 touching neighbour.
  18. After that, place a marker at E5. Region has exactly one open cell left. Placing here clears the rest of row 5, column E, and its region.
  19. Now, place a marker at D8. Region has exactly one open cell left. Placing here clears the rest of row 8, column D, and its region.
  20. Finally, place a marker at B3. Region has exactly one open cell left.

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