[Retros] die Schwalbe 13237 - Gerald Irsigler

[raosorio at fibertel.com.ar]

13237 - 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? 

I understand that the author assumes: a) no pawns on the 1st/8th rank
b) one white king and one black king on the board.

The first obvious restriction is that squares in contact are not posible to
avoid kings in contact.

But the hardest restrictions appear to avoid double illegal checks,

1) Double check over the same king: no double geometrical relation
between one square and the others (orthogonal, diagonal and 2,1 knight type,
unless some magic discovered check position appears)

2) Both kings under check (I'm having polemics enough in this respect due to
our Die Schwalbe problem 13071):  just one couple of squares in checking
position (orthogonal, diagonal or 2,1 knight type). Otherway, both kings could
be placed under check.

Based on this,  apparently  6 squares is the maximum (this is not a strict conclussion,
it's what I found just "moving the hands").