[Retros] Example 4 (better version of E.P.) Yefim 08/23/2004

[Mario Richter]

Hello,

andrew buchanan wrote:


> Art 9.2: "Positions are not the same if a pawn that could have been

> captured en passant can no longer be captured."

>  

> I think that is pretty unambiguous. And if an e.p. is illegal because

> it would leave/put the player in check, then the pawn can't be captured

> by e.p. So the positions are the same.


I do agree that the positions are the same, but I do not agree
that the en passant part of article 9.2 is "pretty unambiguous".

How about the following position:

7k/2p5/8/3P4/1P6/8/8/7K b - - 0 1

white: Kh1 Pb4 Pd5
black: Kh8 Pc7
black to move

Black plays 1. ... Pc7-c5
The game continues: 2. Kh1-g1 Kh8-g8 3.Kg1-h1 Kg8-h8

The black pawn c5, that could have been captured e.p. after black's first
move, still can be captured after black's third move.
So, according to the above rule, are the positions after 
black's first and black's third move the same?


BTW, the definition of two positions being the same was discussed
already in an article by Joerg Kuhlmann in "Die Schwalbe", February 1994,

where the author gave some positions, which are quite similiar to those
given by Yefim Treger:

* Right to castle, but no possiblity to castle:
  r3k3/8/8/3N4/8/8/8/7K w q - 0 0
  white: Kh1 Nd5
  black: Ke8 Ra8 
  and black still having the right to castle:

  Is the position after 1.Nc7+ the same as after
  1. ... Kd8 2.Nd5 Ke8 3.Nc7+?
  (Kuhlmann says yes.)

* Right to castle, but no possiblity to castle:
  4k2r/4P3/8/8/8/8/8/4K2b w k - 0 0
  white: Ke1 Pe7
  black: Ke8 Rh8 Bh1

  Is this position the same as after 1. Kf2 Kf7 2.Ke1 Ke8?
  (Kuhlmann says yes.)

* Technical prepositions for e.p. capture fullfilled, but 
  e.p. capture not legal:

  8/8/8/8/2kp4/8/4P3/4KB2 w - - 0 0
  white: Ke1 Bf1 Pe2
  black: Kc4 Pd4

  White plays 1. e4+
  Is the resulting position the same as after
  1. ... Kc3 2. Ke2 Kc4 3.Ke1+ ?
  (Kuhlmann says yes.)
  
* Starting from the initial postion, is the position after 1.e4 e5 
  the same as after 2.Ke2 Ke7 3.Ke1 Ke8?
  (Kuhlmann says no.)

Kuhlmann's criteria for comparing two positions is
essentially the same as that mentioned by Michael Niermann-Rossi:


> two positions are the same, if the trees starting from the positions are

> identical.



Here is my attempt to tranlate this rather mathematical definition
into a more non-mathematical language:

Two positions are the same, if 

(1) the same player has the move, 
(2) pieces of the same kind and colour occupy the same squares, 
(3) the set of all legal moves of the side to move is the same
    in both positions
and 
(4) the future castling capabilities of both sides are the same
    in both positions.

Where "future castling capabilities" is defined as follows:

In a given position, a side has the future castling capability 
to castle king-side (resp. queen-side), if neither his king 
nor his king-side rook (resp. queen-side rook) has ever moved 

 AND

there exists at least one sequence of moves from the given position, 
in which the castling can actually be executed.



With this definition, in the following (slightly modified) example
given by Michael Niermann-Rossi, black has no future castling capabilities:

1bB1k2r/1P2p3/4P3/8/8/1q6/8/K7 w k - 0 0
white: Ka1 Bc8 Pb7 Pe6
black: Ke8 Qb3 Rh8 Bb8 Pe7
 
so this posotion would be considered the same as after
1. Bd7+ Kd8 2.Bc8 Ke8

 
greetings
  mario