14
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I assume most people on PSE know a bit of chess

With white to move, this is checkmate (1. Qb7#):

Checkmate

That's boring. Let's try something more interesting. With black to move, black can FORCE white to checkmate (1... Qxb7+ 2. Qxb7#):

Force checkmate

We can go another step. In this position, white can force black to force white to checkmate (1. Qxb7+ Qxb7+ 2. Qxb7#):

Force forcing checkmate

I'm going to call this a forced checkmate in 2 (an abuse of tradition puzzles where you have to force checkmate in 2).

I can do forced checkmate in 12 (1... Qxb7+ 2. Qxb7+ Qxb7+ 3. Qxb7+ Qxb7+ 4. Qxb7+ Qxb7+ 5. Qxb7+ Qxb7+ 6. Qxb7+ Qxb7+ 7. Qxb7#):

Forced checkmate in 12

Can you do better?

NOTES:

  • There must be exactly one line after the first move, otherwise it is not strictly forced and I have to think too much.
  • The position must be theoretically possible (so not 10 queens of one colour, 11 bishops of one colour, etc.)
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  • 3
    $\begingroup$ @humn you can just put s (small), m (medium), l (large and default), or h (huge) after the 5 letter imgur address. m works best for me $\endgroup$ – Beastly Gerbil Mar 9 '17 at 21:59
  • $\begingroup$ This puzzle must have been fun to concoct, amounting to the creation/solution of these examples and others along the way $\endgroup$ – humn Mar 10 '17 at 1:46
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    $\begingroup$ related, cross-site dupe: chess.stackexchange.com/q/8789/9025 $\endgroup$ – Herb Wolfe Mar 10 '17 at 2:38
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    $\begingroup$ @ais523 But selfmates can have multiple branches. Also, selfmates would only be forced checkmate in (odd #). $\endgroup$ – boboquack Mar 10 '17 at 23:21
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    $\begingroup$ I have a question - why not 2 ... Qbxb7? $\endgroup$ – Wen1now Mar 11 '17 at 7:31
3
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Yes, I can do better:

I can do forced checkmate in 13 (1... Qbxb7+ 2. Qxb7+ Qxb7+ 3. Qxb7+ Qxb7+ 4. Qxb7+ Qxb7+ 5. Qxb7+ Qxb7+ 6. Qxb7+ Qxb7+ 7. Qxb7 Qxb7#): enter image description here

(sorry for stealing your answer)

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1
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In this position that I composed long ago (this position is legal), every move is completely forced and it is checkmate in

!8 moves (15 ply): 1. Qxc7+ Qxc7+ 2. Qxc7+ Qxc7+ 3. Qxc7+ Qxc7+ 4. Bxc7+ Kb7+ 5. Bb8+ Qxe7+ 6. Qxe7+ Qxe7+ 7. Qxe7+ Qxe7+ 8. Rxe7#

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  • $\begingroup$ I alrighty have you beat: sorry! $\endgroup$ – Rewan Demontay Jun 25 at 22:46
  • $\begingroup$ And this question is about a selfmate:You could add in the black queen moving to her square for 16 like mine got a tie. $\endgroup$ – Rewan Demontay Jun 25 at 22:48
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    $\begingroup$ Also, funnily enough, before I even knew you were on SE just now, I cited your chess com forum post with this position in this CSE question, =D: chess.stackexchange.com/q/8789/15543 $\endgroup$ – Rewan Demontay Jun 25 at 22:51
  • $\begingroup$ Hah that's really interesting. Yes I could place the black queen on b7 for a selfmate but I just posted the position as originally intended (all moves forced). It was really cool to see the 16 ply forced checkmate, maybe I'll try to tie that one day :) $\endgroup$ – shoopi Jun 26 at 9:19
1
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EDIT: Actually, seeing the "only one line" requirement, here is a 16 that is a slight modification of a position made by Bernd Schwarzkopf and Karl Scherer (Urdruck) im Feensnach 1980 on page 13. A PDF can be found here.

! FEN: 1KN4r/r7/k7/pr2R3/5b2/8/8/8 w KQkq - 0 9 (Picture Please!)

! 1. Qxa7+ Qxa7+ 2. Rxa7+ Qxa7+ 3. Rxa7+ Qxa7+ 4. Rxa7+ Qxa7+ 5. Rxa7+ Qxa7+ 6. Rxa7+ Qxa7+ 7. Rxa7+ Rxa7 8. b5+ Rxb5#

Apronus Link


EDIT: THIS SECTION IS WRONG

If we modify Alex Khanayn's forced checkmate in 22 ply record from Journal Entry #267 on Tim Krabbe's wesbite, we can raise the bar to 21.

You can replay the moves here in Apronus

! enter image description here ! ! 1. Rf3+ Nh3f4+ 2. Rgxf4+ Ng2xf4+ 3. Rhxf4+ N2xf4+ 4. Rfxf4+ Ndxf4+ 5. Rxf4+ Nexf4+ 6. Rxf4+ Ngxf4+ 7. Rxf4+ Nxf4+ 8. Rxf4+ Kxf4+ 9. Qce5+ Qxe5+ 10. Qhxe5+ Rexe5+ 11. Qxe5+ Rxe5#

I also want to show my own personal compositions for forced seflmates that involve castling, en passant, and promotion.

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    $\begingroup$ This doesn't seem to satisfy the condition that there's exactly one line. $\endgroup$ – greenturtle3141 Jun 25 at 21:53
  • $\begingroup$ Yeah I saw. My edit has been on hold until now. I have fixed my answer. $\endgroup$ – Rewan Demontay Jun 25 at 22:41

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