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Here's an interesting twist on the theme of chess puzzles, originally from the book Caissa's Fairy Tales by T. R. Dawson.

board

What is the minimum number of moves White needs to win this game from the position shown above (it is White's turn to play), given that pieces cannot move while guarded (i.e. protected by other pieces of the same colour)?

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    $\begingroup$ @stackreader No, the king can't move into check even if it would be checked by a 'guarded' piece. (If you like, think of it like this: the possibility of capturing the black king and winning the game outweighs any other considerations for a white piece.) $\endgroup$
    – Rand al'Thor
    Commented Oct 14, 2016 at 2:56
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    $\begingroup$ I would suggest adding that comment to the question as it is a big assumption to make that gaurded pieces cant move but they can still prevent the king moving into "checked" positions. $\endgroup$
    – gtwebb
    Commented Oct 14, 2016 at 3:15
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    $\begingroup$ Nice question. @justhalf The issue raised by stack reader & gtwebb is worth answering explicitly; given that the rules of chess are being altered, the alteration's scope might not be obvious to all solvers. Indeed some fairy conditions do affect check -- e.g. in Madrasi, paralysed units may not move or check. $\endgroup$
    – Rosie F
    Commented Oct 14, 2016 at 10:57
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    $\begingroup$ If guarded pieces cannot move AND don't threaten, the Black King escapes on his first move past the Rook and White cannot win (further moves failing to free his pieces sufficiently). The problem infers that White can win and that the problem is minimizing the amount of moves to do so, therefore it's safe to infer that the pieces do threaten even when guarded. It would be interesting to see another problem with those rules, but this isn't it. $\endgroup$
    – Danikov
    Commented Oct 14, 2016 at 12:59
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    $\begingroup$ I feel like this question should be clarified to mention that the king is not under the restriction that "pieces which are guarded cannot move". $\endgroup$
    – Shufflepants
    Commented Oct 14, 2016 at 13:58

5 Answers 5

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The answer:

4 moves
C4,Kc5, Kb5, Ra7
you need to block the rook from protecting the king, so the king can block the rook from protecting the rook. At which point the second rook can move to check mate.

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    $\begingroup$ The black king would be in a position to capture a piece and stay gaurded before the rook can move to a7. Or the king could easy move behind the rook or pawn. $\endgroup$
    – gtwebb
    Commented Oct 14, 2016 at 2:44
  • $\begingroup$ Wouldn't it be faster to just do kc5, kb5, Ra7, or am I missing something? $\endgroup$
    – Matthew0898
    Commented Oct 14, 2016 at 4:33
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    $\begingroup$ @Matthew0898 I'm assuming he's counting the b4 rook as "guarding" the king. Hence why c4 is necessary. $\endgroup$
    – Dennis Meng
    Commented Oct 14, 2016 at 5:23
  • $\begingroup$ @DennisMeng Yeah, I totally missed that. Thanks! $\endgroup$
    – Matthew0898
    Commented Oct 14, 2016 at 5:34
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4 moves

1. c4 Ka2 {Only legal moves} 2. Kc5 Ka3 {Black moves to stall inevitable} 3. Kb5 Ka2 {White king frees b7 rook to move} 4. Ra7 #

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  • $\begingroup$ Why would 3. Kb6 be illegal? $\endgroup$
    – user2390246
    Commented Oct 14, 2016 at 10:07
  • $\begingroup$ @user2390246 It's not illegal, but losing the rook lead to stalemate, since after black captures the rook, the pawn would be the only legal piece to move. After c5, white would no longer have any moves as every piece would be guarded. $\endgroup$
    – user4221
    Commented Oct 14, 2016 at 10:51
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    $\begingroup$ Sorry, I meant to say, why would 3. Kb5 be illegal? $\endgroup$
    – user2390246
    Commented Oct 14, 2016 at 12:35
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    $\begingroup$ @user2390246 No reason. Answer fixed. $\endgroup$
    – user4221
    Commented Oct 15, 2016 at 6:15
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Going hamateur has a good answer but I will go with the possibility that the pawn is actually in the enemy zone and not initial position.(I might be wrong but it was not mentioned what side of the board the white player started)

It's possible in 2 moves. pawn goes down 1 and transform in a queen, then move to b2 for checkmate.

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    $\begingroup$ The letters are included on the diagram in the question. White always starts from rank 1 and Black from rank 8. $\endgroup$
    – Rand al'Thor
    Commented Oct 14, 2016 at 2:00
  • $\begingroup$ I see, I didn't know that. Thanks for the info. Does no accepted answers yet mean there is still a better answer out there? 4 move is the best I could find so far. Should I keep looking? $\endgroup$
    – stack reader
    Commented Oct 14, 2016 at 2:07
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    $\begingroup$ Generally, no accepted answers yet just means I'm slow at accepting answers :-) I'm pretty sure GH's answer is optimal. $\endgroup$
    – Rand al'Thor
    Commented Oct 14, 2016 at 2:19
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OP mentioned that

pieces cannot move while guarded

In such a case both rooks can't move because they are guarding each other. Unless OP meant that pieces can't move to such a position where they can check the Black King while being guarded by the teammates (like the rook at b4 can't move to b2 because it is being guarded by the rook at b7). Considering latter to be the case...

The minimum number of moves is

1

with two possibilities either

rook at b4 moves a4

or

rook at b7 moves to a7

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  • $\begingroup$ But the rooks can't move! Just like you said, initially both rooks are guarding each other. This isn't doable in one move. $\endgroup$
    – Rand al'Thor
    Commented Oct 14, 2016 at 20:03
  • $\begingroup$ @randal'thor ok so former is the case, but then we will never have those rooks moving because they are always guarding each other. Even the king can't move because he is being protected by rook at b4.The only unguarded member is the pawn. White won't win this way. $\endgroup$
    – Joy Grewal
    Commented Oct 14, 2016 at 20:14
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    $\begingroup$ Yes, the rooks can't move until something comes between them, and the king can't move until something comes between it and the rook at b4. To see how White wins, you can read Going hamateur's answer. $\endgroup$
    – Rand al'Thor
    Commented Oct 14, 2016 at 20:19
  • $\begingroup$ @randal'thor You are right, I thought it over, the king would have to cut off the rooks from each other, also king is not protected by the rook he is rather being supported by the rook at b4. It was a nice puzzle. Cheers!!! $\endgroup$
    – Joy Grewal
    Commented Oct 15, 2016 at 6:24
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THIS ANSWER ASSUMES THAT IF A PIECE CANT MOVE IT CAN'T THREATEN

With this assumption

White cant win

If somebody could help me embed a chess game that would help to visualize but white basically has two options

Option 1

Get king to B5 to free rook at B7 or

Option 2

Get king to B6 to free rook at B4.

First 2 turns for white are

c4, Kc5

First 2 turns for black are

Ka2, Ka3

Now

White moves KB5, Black takes the rock at B4, since the King is frozen by the pawn this is a legal move and black has check mate.

Or

White moves Bb6 and again black takes rook at B4. This leaves only the white pawn free as the king and rook cannot be separated so black wins again.

And

Since white eventually has to free one of the rooks to win, black can just stay within one square of B4 and he can win as outlined above regardless of any other delaying moves the white king makes.

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  • $\begingroup$ How would the black king capture the b4 rook, if it's protected? $\endgroup$
    – Herb
    Commented Oct 14, 2016 at 2:04
  • $\begingroup$ @HerbWolfe I rewrote to make it clearer. The king can capture the rook at B4 because the pawn is guarding the king (so the king can't move). You could argue this means the rook is unguarded but it doesn't really matter because he doesn't have a chance to move before the black king captures the rook. $\endgroup$
    – gtwebb
    Commented Oct 14, 2016 at 2:29
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    $\begingroup$ A king can't move into a square which is guarded/threatened by other piece, regardless whether that piece can move or not. (think of discovered check, where a piece can't move because that will cause the king to be checked. The enemy king still can't move into the square threatened by the piece) $\endgroup$
    – justhalf
    Commented Oct 14, 2016 at 3:51
  • $\begingroup$ Could I ask why the downvotes? Given the information in the question assuming a gaurded piece cant check seems valid to me in which case i believe this is the right answer. $\endgroup$
    – gtwebb
    Commented Oct 14, 2016 at 3:53
  • $\begingroup$ @justhalf not sure what open check means if the piece cant move i dont see any reason why it should be able to check the king. White has no legal way to attack the square the king moves to. $\endgroup$
    – gtwebb
    Commented Oct 14, 2016 at 3:55

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