How fast can checkmate occur by just moving the king? How short can a game be in which (after the first move) one player moves only their king, and yet checkmates their opponent?
This is assuming that the players cooperate, i.e. this is a kind of &quot;helpmate.&quot; This generalizes a problem by Stanley Rabinowitz, 1596 Journal of Recreational Mathematics 1987. No peeking at his solution, please! :D
Here is a starter solution in 7 moves.
Can we do better?
[FEN &quot;&quot;]

1.g3 e5 2.Bh3 Ke7 3.Bxd7 Kxd7 4.Kf1 Ke7 5. Kg2 Ke6 6.Kh3 Kf6+ 7.Kh4 Kg6#

If this is best, interestingly, the play is almost forced. The only variations are 4... Kd6 or 4... Ke7 in the line above. There is also 4... Ke6 and 5... Kf5, or 3. Kf1 Kd6/Ke8 4. Bxd7 Kxd7 5.Kg2 Ke6, etc. Only five possible move orders with such a minimal stipulation would be quite nice.