[math-fun] Chess -- mark legal squares

Published in Die Schwalbe, Heft 222, december 2006 by Gerald Irsigler: How many squares can you mark at most on a chess board so that every possible position with pieces on those squares (including wK and bK) is legal? (e.g.: mark e1 g2 h1 isn't valid, since wKe1, wBh1 wKg2 isn't legal. e1 g3 h1 is valid though) Best, É.