QED Logic quod erat demonstrandum
Logic Ascent

How to solve this 8×8 logic puzzle

8×8 · steady · 17 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 842). Solving it from scratch takes 27 logical steps, using 6 techniques: Single cell, Row/column exclusion, Adjacency clear, Region confinement, Paired regions, Shape squeeze. Every step below is forced: nothing here is a guess.

  1. First, 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.
  2. Next, 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.
  3. Then, every open cell left in this region sits in column H, so its marker has to land there; that clears every other open cell in the column.
  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, together, two regions have open cells only in columns E and F; with one marker each, those two markers have to fill exactly those two columns, so every other region's open cells there can be cleared.
  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 remaining candidate in this region touches C3 and D3, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
  8. First, every remaining candidate in this region touches B4, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
  9. Next, every remaining candidate in this region touches G6, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
  10. Then, place a marker at G7. Region has exactly one open cell left. Placing here clears the rest of row 7, column G, and its region. Placing here also clears its 1 touching neighbour.
  11. After that, place a marker at H5. Region has exactly one open cell left. Placing here clears the rest of row 5, column H, and its region.
  12. Now, place a marker at E1. Column 5 has exactly one open cell left. Placing here clears the rest of row 1, column E, and its region. Placing here also clears its 1 touching neighbour.
  13. From there, 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.
  14. Following that, 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.
  15. First, place a marker at F4. Region has exactly one open cell left. Placing here clears the rest of row 4, column F, and its region.
  16. Next, place a marker at B8. Region has exactly one open cell left. Placing here clears the rest of row 8, column B, and its region.
  17. Finally, place a marker at D6. Region has exactly one open cell left.

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