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Subject: 'Broken' index positions, legal positions and unreachable positions

Author: GuyHaworth

Date: 07:20:45 02/04/05

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The 'broken' index positions in Eugene's index-scheme are for:

- positions where the side _not_ to move is in check, and/or
- two men are 'placed' on one square, or
- the index position is not used

As noted elsewhere, the number of index positions is (in some cases, crucially)
reduced by avoiding creating checks not blockable by the placement of further
men on the board.


Are the the positions cited legal but unreachable from the conventional starting
position for (Western) Chess?


In some variants of W Chess, e.g. FRC as cited, the positions may be of
interest.

Anyway, spotting all the unreachable positions is hard, and therefore comparison
of one EGT's stats with another's is less likely.


Where the unreachables often show up is in maximal positions, e.g. KQPKQP with
the P giving check from its home square, see P. Karrer's ICGA paper post the
Kasparov-World game.

g



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