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 556). Solving it from scratch takes 22 logical steps, using 5 techniques: Single cell, Row/column exclusion, Adjacency clear, Region confinement, Line confinement. Every step below is forced: nothing here is a guess.

  1. First, 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.
  2. Next, place a marker at B5. Region has exactly one open cell left. Placing here clears the rest of row 5, column B, and its region. Placing here also clears its 3 touching neighbours.
  3. Then, place a marker at A3. Region has exactly one open cell left. Placing here clears the rest of row 3, column A, and its region.
  4. After that, 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.
  5. Now, place a marker at F2. Region has exactly one open cell left. Placing here clears the rest of row 2, column F, and its region. Placing here also clears its 1 touching neighbour.
  6. From there, every open cell left in this region sits in row 4, so its marker has to land there; that clears every other open cell in the row.
  7. Following that, every open cell in column H 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, place a marker at G6. Column 7 has exactly one open cell left. Placing here clears the rest of row 6, column G, and its region. Placing here also clears its 1 touching neighbour.
  9. Next, place a marker at H8. Region has exactly one open cell left. Placing here clears the rest of row 8, column H, and its region.
  10. Then, place a marker at C7. Region has exactly one open cell left. Placing here clears the rest of row 7, column C, and its region.
  11. After that, place a marker at D1. Region has exactly one open cell left. Placing here clears the rest of row 1, column D, and its region.
  12. Finally, place a marker at E4. Region has exactly one open cell left.

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