[Retros] Andrew's problem

[Nicolas DUPONT]

>R162 - Andrew Buchanan

>r1bqkbn1/ppppp1p1/5p2/8/6pP/4P3/PPPPP1P1/1RBQKB1R (14+14)

>+---+---+---+---+---+---+---+---+

>|*R | . |*B |*Q |*K |*B |*N | . |

>+---+---+---+---+---+---+---+---+

>|*P |*P |*P |*P |*P |   |*P |   |

>+---+---+---+---+---+---+---+---+

>|   | . |   | . |   |*P |   | . |

>+---+---+---+---+---+---+---+---+

>| . |   | . |   | . |   | . |   |

>+---+---+---+---+---+---+---+---+

>|   | . |   | . |   | . |*P | P |

>+---+---+---+---+---+---+---+---+

>| . |   | . |   | P |   | . |   |

>+---+---+---+---+---+---+---+---+

>| P | P | P | P | P | . | P | . |

>+---+---+---+---+---+---+---+---+

>| . | R | B | Q | K | B | . | R |

>+---+---+---+---+---+---+---+---+

>Position after white's 12th move. Black may claim a draw now.

>Proofgame?


Dear Joost,

Black may claim a draw only if its 12th move can repeat the same position
for the third time.

On the black side, a rook or a king move will loose the right to 0-0-0.
Hence the repeated position can only appear after 12)... Ch6.

On the white side, a rook'h1 or a king move will loose the right to 0-0.
Hence only moves from rook'b1 can (will) achieve the possible draw.

It thus remains to show that the position after 12)... Ch6 can be obtained
in fact after 8)... Ch6. The unique possible sequence is 1) Ca3 Cc6 2) Cc4
Ce5 3) Cxe5 h5 4) Cg4 hxg4 5) Ch3 Txh3 6) Tb1 Te3 7) fxe3 f6 8) h4 Ch6.

A very nice and clever problem by Andrew.

Best regards. Nicolas.