QED Logic quod erat demonstrandum
Logic Ascent

How to solve this 7×7 logic puzzle

7×7 · steady · 12 steps

A
B
C
D
E
F
G
1
2
3
4
5
6
7
1
2
🐱
🐱
3
4
🐱
🐱
5🐱
6🐱
7🐱

This 7×7 board rates as steady (difficulty score 926). Solving it from scratch takes 20 logical steps, using 6 techniques: Single cell, Row/column exclusion, Adjacency clear, Region confinement, Line confinement, Forced chain. Every step below is forced: nothing here is a guess.

  1. First, 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.
  2. Next, place a marker at E6. Region has exactly one open cell left. Placing here clears the rest of row 6, column E, and its region. Placing here also clears its 2 touching neighbours.
  3. Then, place a marker at C5. Region has exactly one open cell left. Placing here clears the rest of row 5, column C, and its region. Placing here also clears its 2 touching neighbours.
  4. After that, place a marker at B7. Region has exactly one open cell left. Placing here clears the rest of row 7, column B, and its region.
  5. Now, marking A1 would force D3 (the last open cell in region 4), and then column F would be left with no open cell for its marker. So A1 can't be the marker there.
  6. From there, marking D1 would force G2 (the last open cell in region 2), and then row 3 would be left with no open cell for its marker. So D1 can't be the marker there.
  7. Following that, 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.
  8. First, every open cell in row 1 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.
  9. Next, place a marker at D3. Column 4 has exactly one open cell left. Placing here clears the rest of row 3, column D, and its region.
  10. Then, place a marker at A2. Region has exactly one open cell left. Placing here clears the rest of row 2, column A, and its region.
  11. After that, place a marker at G4. Region has exactly one open cell left. Placing here clears the rest of row 4, column G, and its region.
  12. Finally, place a marker at F1. Region has exactly one open cell left.

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