9
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From the chess starting setup position, the twins are playing White.

All regular rules are applied except:

  • The Black player wins if he makes it to the 7th move
  • Each twin makes a move, then black moves

How can the twins checkmate their opponent to win?

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  • $\begingroup$ Black moves when the King is in check. $\endgroup$ – TSLF Jun 4 '17 at 17:22
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    $\begingroup$ So, white has 2 moves for every black 1? $\endgroup$ – Beastly Gerbil Jun 4 '17 at 17:24
  • $\begingroup$ yes,when the first white move is not a check $\endgroup$ – TSLF Jun 4 '17 at 17:26
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    $\begingroup$ Does the checkmate condition account for white getting two moves in a row, or do the twins have to enter a position that would be checkmate under standard rules? $\endgroup$ – xnor Jun 4 '17 at 22:07
7
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Targeting the weakest square (f7) is the quickest.

I have the following solutions, using standard algebraic notation, with white move 1, white move 2; black move

1. Nf3, Ne5; f6 (or f5)
2. e4, Qh5+; g6
3. Nxg6, Ne5#

If 1. ... Nh6 (to protect f7)
2. e4, Bc4; e6

If 2. ... d5, then 3. Bxd5, Bxf7, continuing as below,
while on 2. ... f6 (or f5), 3. Bf7+; Nxf7 4. Qh5, Qxf7#

3. Bxe6, Bxf7+; Nxf7
4. Qf3, Qxf7#

Blocking the diagonal first with 1. ... e6 gives us:
2. e4, Qh5; Nh6
3. Nxf7, Nxh6+ (or Nd6+); Ke7
4. e5, Qf7#

if 2. ... Ke7
3. Qxf7+; Kd6
4. Na3, Nac4+; Kc5
5. Qxe6, Qd5+; Kb4
6. Qa4#

1. ... g6 is no better, with play as in other lines.
2. e4, Bc4; Nh6

2. ... e6 allows 3. Qf3, Qxf7#

3. Qf3, Bxf7+; Nxf7
4. Qxf7+

While 1. ... d6, threatening the Knight, fails to
2. e4, Bb5+; Qd7
3. Qf3, Qxf7+; Kd8
4. Qxf8+; Qe8
5. Qxe8#

1. ... c6, trying to give the king an escape route
2. e4, Qf3; Qa5
3. Qxf7+; Kd8
4. Qxf8+; Kc7
5. Nf7, Qd8#

It appears the best black can do, is get mated at move 7.

1. Nf3, Ne5; Nc6
2. Nxc6, Ne5; e6, continuing as above.
3. e4, Qh5; Ke7
4. Qxf7+; Kd6
5. Na3, Nac4+; Kc5
6. Qxe6, Qd5+; Kb4
7. Qa4#

Or, with 3. ... Qe7, as OP suggests,
4. Nxf7, Nd6+; Kd8
5. d4, Nf7+; Ke8 (if Qxf7, 6. Qxf7, Qxf8#)
6. Bg5, Nd6+; Kd8
7. Qe8#

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  • $\begingroup$ does that count as making it to move 7? $\endgroup$ – Destructible Lemon Jun 5 '17 at 0:37
  • $\begingroup$ I assumed that black has to make it to their move 7. $\endgroup$ – Herb Wolfe Jun 5 '17 at 0:39
  • $\begingroup$ From the last move line above,with queen guarding f7.. black might make it to 7th 1. Nf3, Ne5; Nc6 2. Nxc6, Ne5; e6, continuing as above. 3. e4, Qh5;Qe7(protecting f7) 4. d4,Bg5; g6 5. Bxe7,Qf3; Nh6.. 6. Bxf8,Bxh6;Rf8 7. Bxf8,Qxf7+;Kd8! $\endgroup$ – TSLF Jun 5 '17 at 15:49
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    $\begingroup$ @TSLF White mates with 7. Qa3, Qxf8#. 6. ... f5 or f6 may be better for black. I'm still looking at it, though. $\endgroup$ – Herb Wolfe Jun 5 '17 at 17:08
  • $\begingroup$ @Herb Wolfe-at 1st looks like theknight may be there too soon that it can threaten twice by black: 1. Nf3, Ne5; Nc6 2. Nxc6, Ne5; d6 3. e4, Qh5; but black cant take e5, 3..,..Be6..4. Bc4,Bxe6.. $\endgroup$ – TSLF Jun 6 '17 at 17:49

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