[Retros] Again about En Passant Chess

Noam Elkies elkies at math.harvard.edu
Sun Mar 25 11:53:10 EDT 2007

I wrote:

>> I don't think this visualization works, because the opponent may

>> have a choice of which billiard ball to activate, and indeed

>> may choose to activate none if the opponent's piece stopped

>> short of its target [...]

>> A nice aspect of your rule that I didn't notice before is that

>> it incorporates the rule forbidding a King from moving through check

>> when castling!

Franco replies:

> you are right: no paradoxes.

> The only one could arise if we allow to a piece being captured

> to capture the piece is just going to capture it before that happen!

> If so, who will be captured?

Hm... That looks like a good case for incorporating the condition
implicit in Guus Rol's observation:

| A question to ponder on is whether or not to allow for e.p. captures

| in the same line of action, e.g. wBd2-h6, bBc1xf4 e.p. Though there

| is no iron-clad argument to reject the validity of this type of e.p.

| action, there is a feel that the en passant concept should include the

| property of "cross-firing" which is missing in this type.

Excluding e.p. captures on the same line would automatically take care
of this paradox too.

> But, what we have to do with Knight rules? It just flights

> when it moves, so it could'nt be captured en passant. Right?

That makes sense. Pieces that can be captured e.p. are "line-pieces",
whose path can be obstructed by a man (of either color), and can now
be captured e.p. on any of the potential obstruction squares:
Queen, Rook, Bishop, and also Pawn making a double-move and
King in the process of castling. The Knight is a "leaper",
which cannot be obstructed and should not be subject to e.p. capture.
(A Chinese knight, on the other hand, *can* be obstructed, and thus
should be e.p. capturable on the squares it moves through on the way
to its destination.)

This still leaves the question of whether, after the sequence
1 X:Y, Z:X e.p.; the piece Y should reappear on the board --
and if we answer Yes then do we let Kings be reincarnated in
the same way.


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