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
3
4
🐱
🐱
🐱
5
6
🐱
7
8
🐱
🐱
🐱

This 8×8 board rates as steady (difficulty score 825). Solving it from scratch takes 25 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 2, 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 3, so its marker has to land there; that clears every other open cell in the row.
  3. Then, every remaining candidate in this region touches G1 and H1 and G3 and H3, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
  4. After that, place a marker at F3. Region has exactly one open cell left. Placing here clears the rest of row 3, column F, and its region. Placing here also clears its 3 touching neighbours.
  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, place a marker at G8. Region has exactly one open cell left. Placing here clears the rest of row 8, column G, and its region.
  7. Following that, 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.
  8. First, every open cell left in this region sits in row 7, so its marker has to land there; that clears every other open cell in the row.
  9. Next, 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.
  10. Then, every remaining candidate in this region touches D4 and D6 and E6, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
  11. After that, every remaining candidate in this region touches C7, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
  12. Now, place a marker at E7. Region has exactly one open cell left. Placing here clears the rest of row 7, column E, and its region.
  13. From there, place a marker at D5. Region has exactly one open cell left. Placing here clears the rest of row 5, column D, and its region. Placing here also clears its 2 touching neighbours.
  14. Following that, place a marker at B6. Region has exactly one open cell left. Placing here clears the rest of row 6, column B, and its region.
  15. First, place a marker at A4. Region has exactly one open cell left. Placing here clears the rest of row 4, column A, and its region.
  16. Finally, place a marker at C1. Region has exactly one open cell left.

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