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You enter a chess club and see pieces arranged on a board like this:

enter image description here

Someone tells you that 2 players were playing a game (according to standard chess rules). At the end white announced a mate in 4, which caused the black player to angry leave the room shortly followed by the white player.

How can white win the game in 4 moves?

This puzzle is one of Lord Dunsany’s Inferential Problems.

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  • $\begingroup$ Are you sure this is possible? I have a hard time believing a mate can be forced in 4 moves, with best play from both sides. $\endgroup$
    – astralfenix
    Commented Apr 10, 2016 at 10:43
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    $\begingroup$ @astralfenix Yes it is possible. You must use all information available in the puzzle to solve it. $\endgroup$
    – Sleafar
    Commented Apr 10, 2016 at 10:45
  • $\begingroup$ Ok. I take my words back. $\endgroup$
    – klm123
    Commented Apr 10, 2016 at 10:58
  • $\begingroup$ @Sleafar I don't think my comment could be understood by somebody who hadn't figured out the twist yet, but now I have deleted it anyway. $\endgroup$
    – kasperd
    Commented Apr 10, 2016 at 19:01

2 Answers 2

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The twist is that:

we see the board from the black side, i.e., black king is on e1, white king is on d8. Black pawns are one step from promotion, black pieces are almost immobile (only the knights can move at this position).

Now the sequence of white moves is:

1. Nc6 …
2. Nb4 …
3. Nd3#

It can be interrupted by

1. … Nf3
2. … Ne5

In that case white needs one extra move:

3. Qxe5

enter image description here

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  • $\begingroup$ Great puzzle and solution. But seems like it would have required a lot of co-operation between the 2 players to keep the white king alive! Finding the minimum number of moves to achieve this situation could be a new puzzle in itself. $\endgroup$
    – KeyboardWielder
    Commented Apr 10, 2016 at 11:19
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    $\begingroup$ Why doesn't the third black pawn from the left take the knight? Its not check mate $\endgroup$
    – Beastly Gerbil
    Commented Apr 10, 2016 at 13:55
  • $\begingroup$ @BeastlyGerbil All the black pawns have already advanced 5 squares each. So the knight is behind the pawn. $\endgroup$
    – Bolo
    Commented Apr 10, 2016 at 14:39
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    $\begingroup$ I knew something was up because the black king and queen are reversed. Trying to think if there's any legal series of moves that could result in this scenario in the first place - certainly wouldn't happen unless both players were cooperating up to this point. $\endgroup$
    – Darrel Hoffman
    Commented Apr 10, 2016 at 15:24
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    $\begingroup$ @DarrelHoffman, KeyboardWielder: Yes, a lot of co-operation was required up to this point. Which is probably why white player's announcement of a mate in 4 (thus breaking the spirit of co-operation) prompted black player to angrily leave the room. $\endgroup$
    – Bolo
    Commented Apr 10, 2016 at 15:41
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Oh, please no.

The chessboard is upside-down.

So, if White plays

Qd8-d3

and then

Qd3(x)c3 Bc8-f5 Bf5xc2 if Black moves his B1 knight

or

Bc8-f5 Qd3(x)c2 if Black moves his G1 knight

he will always win.

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  • $\begingroup$ You were 18 seconds faster :) It seems that we have different solutions. $\endgroup$
    – Bolo
    Commented Apr 10, 2016 at 10:49
  • $\begingroup$ @Bolo I am not sure mine is entirely correct. I forgot about promotion. $\endgroup$
    – BaSzAt
    Commented Apr 10, 2016 at 10:49
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    $\begingroup$ @Sleafar Unfortunately, I think not. If Black moves his B1 knight and promotes his pawn to another knight, he can still avoid mate by capturing White's queen. $\endgroup$
    – BaSzAt
    Commented Apr 10, 2016 at 11:02
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    $\begingroup$ After rotating the board it would be 1. Qe3 Nf3 2. Qxf3 g1=Q, and black should be winning. $\endgroup$
    – Sleafar
    Commented Apr 10, 2016 at 11:29

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