This article is from the Puzzles FAQ, by Chris Cole email@example.com and Matthew Daly firstname.lastname@example.org with numerous contributions by others.
What is the maximum number of knights that can be put on n x n chessboard
without threatening each other?
n^2/2 for n even >= 4.
Divide the board in parts of 2x4 squares. The squares within
each part are paired as follows:
----- |A|B| ----- |C|D| ----- |B|A| ----- |D|C| -----