Answers.Body: <p>There are some tricky positions, where it looks like there is no solution, but then it turns out white must be able to take en passant. In these positions engines might overlook the mate, because they don't have the information about the last black move, whereas a human can deduce this information by retrograde analysis.</p>
  
  <pre><code>4k2r/8/5B1P/3R1KpP/8/8/8/8 w - - 0 1
  </code></pre>
  
  <p>In this position it is impossible to determine which variation leads to mate in two. It is only possible to show that there must be a mate in two-variation.
  Either black moved the pawn on his last move - then taking en passant leads to mate. Or he moved his king or rook - then king e6 leads to mate, as there is no more castling possible.</p>
  
  <p>Edit:
  Another answer, that is just as irrelevant for all practical purposes:
  As we all know thanks to tablebases, there is a big number of forced checkmates out there, that are far beyond the calculation horizon of any engine. Of course we can use tablebases to detect those mates, but it doesn't take any stretch of the imagination, to acknowledge the existence of forced checkmates over thousands of moves, that aren't now, and likely never will be stored in a tablebase.</p>
  
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2015-01-19T12:55:16.883Z
Answers.Text: Technically (http://xkcd.com/1475/), this indeed answers the question, as the question does not ask for "positions" but "examples" - usually, the definition of "position" also requires the specification of castling or en passant rights.
Answers.UserDisplayName: JiK
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2015-01-19T13:24:21.660Z
Answers.Text: "Technically" the definition of "position" indeed requires the specification of castling or en passant rights. But in this case question and answer were obviously inspired by http://chess.stackexchange.com/questions/8400/is-there-a-checkmate-in-the-next-move
Answers.UserDisplayName: BlindKungFuMaster
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2015-01-19T12:38:22.730Z
Answers.LastActivityDate: 2015-01-19T13:46:24.290Z
Answers.LastEditDate: 2015-01-19T13:46:24.290Z
Answers.UserDisplayName: BlindKungFuMaster
Answers.Body: <p>Positing this answer in addition to the answers/comments on tablebases and endgames with ridiculously long forced mates.</p>
  
  <p>White to move and win (I do not know the author of this study, unfortunately). The engines will tend to fail on this and similar problems. 
  As far as I tried, all recommend to move hxg8=Q. Some show 0.00 and are soon thereafter putting black in a stalemate; some show slight advantage for white and are trying to play the game out by letting black to activate his pieces. Further explanation (and hint to the solution) given in the spoiler block below the diagram.</p>
  
  <pre><code>6r1/p1p1p1pP/P1PpP1P1/8/2P1B3/2K3pp/3P2rq/R5bk w - - 0 1
  </code></pre>
  
  <p>Explanation spoiler:</p>
  
  <blockquote class="spoiler">
    <p> The forced mate is not found because the engines use pruning heuristic. It removes certain branches from search tree, after deeming them as irrelevant to the search result (see: <a href="http://chessprogramming.wikispaces.com/Pruning" rel="noreferrer">http://chessprogramming.wikispaces.com/Pruning</a>). In case of this puzzle, the solution consists of many subsequent sacrifices and its branch tends to be discarded when searching. Note: Possibly, with tweaked parameters and mate-search heuristic when exact number of moves required is provided, they could find the solution, but I have not tried that. </p>
  </blockquote>
  
  <p>Solution spoiler:</p>
  
  <blockquote class="spoiler">
    <p> Depending on black's move choices, a few solutions exist (the idea is always the same, though and up until move 12 the line is always the same, too). Here's an example: 1. hxg8=N d5 2. Bf3 d4+ 3. Kb4 d3 4. Nh6 gxh6 5. g7 h5 6. g8=N h4 7. Nf6 exf6 8. e7 f5 9. e8=N f4 10. Nd6 cxd6 11. c7 d5 12. c8=N dxc4 13. Nb6 c3 14. dxc3 d2 15. Kb3 d1=Q 16. Rxd1 axb6 17. a7 b5 18. a8=Q b4 19. Be2 bxc3 20. Bf1 c2 21. Rc1 f3 22. Qxf3 Bf2 23. Bxg2+#</p>
  </blockquote>
  
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2015-01-20T04:07:18.927Z
Answers.Text: I can not find where is the solution for this puzzle?
Answers.UserDisplayName: Salvador Dali
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2015-01-20T08:30:17.783Z
Answers.Text: @SalvadorDali Added solution in the post.
Answers.UserDisplayName: GloriaVictis
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2015-01-20T14:54:01.407Z
Answers.Text: Where is white's mate after 22...Bc5+ followed by 23...Qg1?
Answers.UserDisplayName: GrizzlyRawrz
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2015-01-20T16:12:48.760Z
Answers.Text: @GrizzlyRawrz I guess I am guilty of posting a problem (from my chess club) and not knowing a solution for it - this was my, apparently not successful, attempt at it. Apologies! Regardless, the remainder of the answer - the motif and comment about engines stands (actually, was part of the talk that evening). I suppose you will have to find the actual solution yourselves!
Answers.UserDisplayName: GloriaVictis
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2015-01-23T11:47:38.300Z
Answers.Text: @GrizzlyRawrz After 22...Bc5+ Stockfish gives a solution for mate in 14. It starts with 23. Kb3 Qg1, 24. Bxg2+ Kh2 25. Rxg1 Bxg1
Answers.UserDisplayName: jva
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2015-01-19T14:42:14.007Z
Answers.LastActivityDate: 2018-06-25T09:38:21.607Z
Answers.LastEditDate: 2018-06-25T09:38:21.607Z
Answers.UserDisplayName: GloriaVictis
Body: <p>Do you know example where recent chess engines (Houdini, Rybka, Komodo, ...) failed to find a forced checkmate.</p>
  
  <p>I guess they must find mate in 1 or 2 all the time but maybe are they failing to be as creative as human some times.</p>
  
Comments.ContentLicense: CC BY-SA 3.0
Comments.CreationDate: 2015-01-19T12:58:52.867Z
Comments.Text: Pretty much any sufficiently complicated endgame tablebase position (many 6-man endings, some 5-man endings and maybe even some long KQvKR endings) would suffice: by tablebases we know that certain positions are forced checkmates (in 30, 50, 100, or even more moves) but engines without the aid of tablebases will not find them in any reasonable time. Do you want to include them or only the ones where human can prove a forced mate without computer help?
Comments.UserDisplayName: JiK
Comments.ContentLicense: CC BY-SA 3.0
Comments.CreationDate: 2015-01-19T16:22:43.830Z
Comments.Text: I get your point about tablebase position, thanks. My question didn't specifically restrict position where we need computer help.
Comments.UserDisplayName: Tanj
Comments.ContentLicense: CC BY-SA 3.0
Comments.CreationDate: 2017-10-31T16:22:31.317Z
Comments.Text: It is a bit tricky, but there is the possibility of a repetition (twofold or threefolds), a lot of chess softwares may allow mates in one. See item 330 in this blog: https://timkr.home.xs4all.nl/chess2/diary_17.htm Ten years after, I'm not sure if most programs have resolve this issue.
Comments.UserDisplayName: Evargalo
Tags: <engines><checkmate>
Title: Forced checkmate examples where engines fail
ThreadText: # Forced checkmate examples where engines fail
  
  (Asked by: username_0 on 2015-01-19)
  
  Do you know example where recent chess engines (Houdini, Rybka, Komodo, ...) failed to find a forced checkmate.
  
  I guess they must find mate in 1 or 2 all the time but maybe are they failing to be as creative as human some times.
  
  **Tags:** engines, checkmate
  
  > **Comments:**
  > **username_1**: Pretty much any sufficiently complicated endgame tablebase position (many 6-man endings, some 5-man endings and maybe even some long KQvKR endings) would suffice: by tablebases we know that certain positions are forced checkmates (in 30, 50, 100, or even more moves) but engines without the aid of tablebases will not find them in any reasonable time. Do you want to include them or only the ones where human can prove a forced mate without computer help?  
  > **username_0**: I get your point about tablebase position, thanks. My question didn't specifically restrict position where we need computer help.  
  > **username_2**: It is a bit tricky, but there is the possibility of a repetition (twofold or threefolds), a lot of chess softwares may allow mates in one. See item 330 in this blog: https://timkr.home.xs4all.nl/chess2/diary_17.htm Ten years after, I'm not sure if most programs have resolve this issue.  
  
  ---
  
  ## 2 answers
  
  ### Answer by username_4 on 2015-01-19. Score: 10 | Accepted answer
  
  Positing this answer in addition to the answers/comments on tablebases and endgames with ridiculously long forced mates.
  
  White to move and win (I do not know the author of this study, unfortunately). The engines will tend to fail on this and similar problems.
  As far as I tried, all recommend to move hxg8=Q. Some show 0.00 and are soon thereafter putting black in a stalemate; some show slight advantage for white and are trying to play the game out by letting black to activate his pieces. Further explanation (and hint to the solution) given in the spoiler block below the diagram.
  
  ```
  6r1/p1p1p1pP/P1PpP1P1/8/2P1B3/2K3pp/3P2rq/R5bk w - - 0 1
  
  ```
  
  Explanation spoiler:
  
  > The forced mate is not found because the engines use pruning heuristic. It removes certain branches from search tree, after deeming them as irrelevant to the search result (see: <http://chessprogramming.wikispaces.com/Pruning>). In case of this puzzle, the solution consists of many subsequent sacrifices and its branch tends to be discarded when searching. Note: Possibly, with tweaked parameters and mate-search heuristic when exact number of moves required is provided, they could find the solution, but I have not tried that.
  
  Solution spoiler:
  
  > Depending on black's move choices, a few solutions exist (the idea is always the same, though and up until move 12 the line is always the same, too). Here's an example: 1. hxg8=N d5 2. Bf3 d4+ 3. Kb4 d3 4. Nh6 gxh6 5. g7 h5 6. g8=N h4 7. Nf6 exf6 8. e7 f5 9. e8=N f4 10. Nd6 cxd6 11. c7 d5 12. c8=N dxc4 13. Nb6 c3 14. dxc3 d2 15. Kb3 d1=Q 16. Rxd1 axb6 17. a7 b5 18. a8=Q b4 19. Be2 bxc3 20. Bf1 c2 21. Rc1 f3 22. Qxf3 Bf2 23. Bxg2+#
  
  > **Comments:**
  > **username_5**: I can not find where is the solution for this puzzle?  
  > **username_4**: @username_5 Added solution in the post.  
  > **username_6**: Where is white's mate after 22...Bc5+ followed by 23...Qg1?  
  > **username_4**: @username_6 I guess I am guilty of posting a problem (from my chess club) and not knowing a solution for it - this was my, apparently not successful, attempt at it. Apologies! Regardless, the remainder of the answer - the motif and comment about engines stands (actually, was part of the talk that evening). I suppose you will have to find the actual solution yourselves!  
  > **username_7**: @username_6 After 22...Bc5+ Stockfish gives a solution for mate in 14. It starts with 23. Kb3 Qg1, 24. Bxg2+ Kh2 25. Rxg1 Bxg1  
  
  ---
  
  ### Answer by username_3 on 2015-01-19. Score: 4
  
  There are some tricky positions, where it looks like there is no solution, but then it turns out white must be able to take en passant. In these positions engines might overlook the mate, because they don't have the information about the last black move, whereas a human can deduce this information by retrograde analysis.
  
  ```
  4k2r/8/5B1P/3R1KpP/8/8/8/8 w - - 0 1
  
  ```
  
  In this position it is impossible to determine which variation leads to mate in two. It is only possible to show that there must be a mate in two-variation.
  Either black moved the pawn on his last move - then taking en passant leads to mate. Or he moved his king or rook - then king e6 leads to mate, as there is no more castling possible.
  
  Edit:
  Another answer, that is just as irrelevant for all practical purposes:
  As we all know thanks to tablebases, there is a big number of forced checkmates out there, that are far beyond the calculation horizon of any engine. Of course we can use tablebases to detect those mates, but it doesn't take any stretch of the imagination, to acknowledge the existence of forced checkmates over thousands of moves, that aren't now, and likely never will be stored in a tablebase.
  
  > **Comments:**
  > **username_1**: Technically (http://xkcd.com/1475/), this indeed answers the question, as the question does not ask for "positions" but "examples" - usually, the definition of "position" also requires the specification of castling or en passant rights.  
  > **username_3**: "Technically" the definition of "position" indeed requires the specification of castling or en passant rights. But in this case question and answer were obviously inspired by http://chess.stackexchange.com/questions/8400/is-there-a-checkmate-in-the-next-move  
  
  ---
  
  