QED Logic quod erat demonstrandum
Logic Ascent

How to solve this 8×8 logic puzzle

8×8 · gentle · 12 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 779). Solving it from scratch takes 23 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, place a marker at A8. Region has exactly one open cell left. Placing here clears the rest of row 8, column A, and its region. Placing here also clears its 1 touching neighbour.
  2. Next, 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.
  3. Then, 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.
  4. After that, place a marker at C2. Region has exactly one open cell left. Placing here clears the rest of row 2, column C, and its region. Placing here also clears its 1 touching neighbour.
  5. Now, place a marker at D7. Region has exactly one open cell left. Placing here clears the rest of row 7, column D, and its region.
  6. From there, every open cell left in this region sits in column E, so its marker has to land there; that clears every other open cell in the column.
  7. Following that, every remaining candidate in this region touches G4, 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 F4 and F5, 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 F3. Column 6 has exactly one open cell left. Placing here clears the rest of row 3, column F, and its region. Placing here also clears its 1 touching neighbour.
  10. Then, 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.
  11. After that, place a marker at H4. Region has exactly one open cell left. Placing here clears the rest of row 4, column H, and its region.
  12. Finally, place a marker at G1. Region has exactly one open cell left.

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