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 807). Solving it from scratch takes 25 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, place a marker at G1. Region has exactly one open cell left. Placing here clears the rest of row 1, column G, and its region. Placing here also clears its 2 touching neighbours.
  2. Next, place a marker at H3. Region has exactly one open cell left. Placing here clears the rest of row 3, column H, and its region.
  3. Then, every open cell left in this region sits in row 8, so its marker has to land there; that clears every other open cell in the row.
  4. After that, 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.
  5. Now, 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.
  6. From there, 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.
  7. Following that, every open cell in column B belongs to the same region, so that region's marker has to be somewhere in this column; that clears every other open cell in the region.
  8. First, every remaining candidate in this region touches A5, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
  9. Next, place a marker at A6. Region has exactly one open cell left. Placing here clears the rest of row 6, column A, and its region. Placing here also clears its 1 touching neighbour.
  10. Then, 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.
  11. After that, every remaining candidate in this region touches D8, 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 D7. Column 4 has exactly one open cell left. Placing here clears the rest of row 7, column D, and its region. Placing here also clears its 1 touching neighbour.
  13. From there, 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.
  14. Following 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.
  15. Finally, place a marker at C2. Region has exactly one open cell left.

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