QED Logic quod erat demonstrandum
Logic Ascent

How to solve this 8×8 logic puzzle

8×8 · steady · 15 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 806). Solving it from scratch takes 26 logical steps, using 5 techniques: Single cell, Row/column exclusion, Adjacency clear, Region 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 row 2, so its marker has to land there; that clears every other open cell in the row.
  3. Then, 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. Placing here also clears its 2 touching neighbours.
  4. After 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 1 touching neighbour.
  5. Now, place a marker at H2. Region has exactly one open cell left. Placing here clears the rest of row 2, column H, and its region.
  6. From there, 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.
  7. Following that, 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.
  8. 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.
  9. Next, 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.
  10. Then, every remaining candidate in this region touches D7, 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 D8. Region has exactly one open cell left. Placing here clears the rest of row 8, column D, and its region. Placing here also clears its 1 touching neighbour.
  12. Now, 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.
  13. From there, 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.
  14. Following that, place a marker at B4. Region has exactly one open cell left. Placing here clears the rest of row 4, column B, and its region.
  15. Finally, place a marker at G5. Region has exactly one open cell left.

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