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Construction challenge:

Find a position with the longest sequence of unique losing moves, i.e. white to move has one and only one move that will lead to a lost (for white) position. White makes this move. Now black by definition has a winning position. But we require there to be one and only one move that will turn that into a loss for black. Etc.

Clarification: If you can give proof that no other move loses eventually that's great but probably quite difficult in practice, so "doesn't lose within ten moves" or so shall suffice.

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Here is a simpler realisation of Glorfindel's solution

enter image description here

Again,

both sides must either mate in 1 or "pass", allowing the opponent to mate in 1. This time the position is legal.

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  • $\begingroup$ You could almost post this as a proof game puzzle ;-) $\endgroup$ Jan 31 at 21:34
  • 2
    $\begingroup$ Doesn't the wording of the question ("sequence of unique losing moves") mean this sequence is only 2? Then it isn't unique anymore. $\endgroup$
    – pkr
    Feb 1 at 23:27
  • $\begingroup$ Unique means unique (i.e., only one losing move, not two) among the options in that turn, not in future turns. $\endgroup$
    – justhalf
    Feb 9 at 8:45
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Here is an idea

which doesn't really work in its current form, because the position is illegal; but it's probably possible to improve it into a working solution

which produces a sequence of length

infinity (if you don't count draw by threefold repetition or the 50 move rule)

enter image description here

White to move has two options; Bb7 (checkmate) and Bc8 (losing). The same holds for Black.

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  • $\begingroup$ Good start! Now what were the rules about those draws again? Do they have to be claimed or are they automatic? $\endgroup$ Jan 31 at 19:55
  • $\begingroup$ They're not automatic, but after 5-fold repetition or 75 moves without pawn-moves or captures the arbiter is required to intervene. $\endgroup$
    – Glorfindel
    Jan 31 at 19:57
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This does not beat infinite

,but 4.5 is possible. Moving the h pawn wins, delaying by playing the king/c-pawn looses (until the c pawn is able to promote). There are no other moves possible.

enter image description here

And another 4.5 enter image description here

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  • $\begingroup$ Yes, I tend to forget that promotion is part of the move, Ill fix it now. $\endgroup$
    – Retudin
    Feb 1 at 11:15

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