QED Logic quod erat demonstrandum
Logic Ascent

How to solve this 8×8 logic puzzle

8×8 · gentle · 13 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 gentle (difficulty score 778). Solving it from scratch takes 24 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 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 2 touching neighbours.
  2. Next, 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. Placing here also clears its 1 touching neighbour.
  3. Then, 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.
  4. After 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. Placing here also clears its 2 touching neighbours.
  5. Now, 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.
  6. From there, 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.
  7. Following that, 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.
  8. First, every remaining candidate in this region touches D8 and E8, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
  9. Next, place a marker at C8. Region has exactly one open cell left. Placing here clears the rest of row 8, column C, and its region. Placing here also clears its 1 touching neighbour.
  10. Then, 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.
  11. After that, place a marker at D3. Region has exactly one open cell left. Placing here clears the rest of row 3, column D, and its region.
  12. Now, 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.
  13. 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. ∎