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I'm starting a new series in which I set a very specific goal with a unique (to be more specifically defined here in a bit) answer in which the solver must construct a chess position, sometimes a game, using given information. My questions are sure to be very defined as to not be close as too broad.

In the event that in turns out that there is an alternate, unintentional solution to my question, I shall heartily accept the interpretation and move on .

I shall now define what a "unique" solution is. Basically, there is really only one way that the solution can be. However, variations of the piece positioning is allowed.

Now let's get to the first challenge! No computers are allowed!.

The task today is to construct four legal chess postions-one for each type of promotion in chess. Each one must be labeled. Assume that White is to be the promoting side and each side plays optimality.

For each diagram, the chosen promotion must be the best move in that position. A motivation for why that promotion is the best move must be stated.

The Task-Try to find the minimum amount of material for each position to ensure that the promotion is the best move.

If you need any clarifications or have a question, feel free to ask in the comments section below! After all, this is a raw and untested idea, so some refinement may be required.

Have fun!

UPDATE: Now that the main question has been solved, I'm here to issue two more related challenges. Try to find the minimal force for positions in which a knight and bishop promotion are the only winning move.

Another challenge is to find a position with a winning knight promotion that has only 4 units.

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  • $\begingroup$ Material being the sum of the piece values (1,3,5,9, etc.) or the number of pieces? $\endgroup$
    – sunfishho
    Commented Jul 11, 2019 at 23:55
  • $\begingroup$ Will ties be broken based on the sum? (aka will 1 queen > 1 rook)? $\endgroup$
    – sunfishho
    Commented Jul 11, 2019 at 23:56
  • $\begingroup$ Sorry for pestering you, but are tablebases allowed? I'm wondering this because I would like to show that a position is a draw because of the 50 move rule. $\endgroup$
    – sunfishho
    Commented Jul 11, 2019 at 23:59
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    $\begingroup$ Is the requirement (1) that the only optimal move be a particular sort of promotion or (2) that the only optimal promotion be a particular sort? #2 seems much easier to arrange than #1. $\endgroup$
    – Gareth McCaughan ♦
    Commented Jul 12, 2019 at 0:24

2 Answers 2

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For Queen:

The only winning move here is to promote to a queen, which skewers the rook. Promoting to a bishop clearly does not lead to a win, and the same holds for a knight. Promoting to a rook leads to a dead draw.
          !


For Knight:

The only drawing move here is to promote to a knight. Promoting to anything else leads to Qh3+ with mate soon to follow.
          2]


For Rook:

Promoting to a queen leads to stalemate, while promoting to a bishop or a knight leads to draw by insufficient material.
          3]


For Bishop:

Promoting to a rook or a queen stalemates, while not promoting loses the pawn and draws by insufficient material. Black can easily draw if White promotes to a knight. However, White can force a mate if they promote to a bishop, because after the bishop moves white plays Ne7! with Bg7# to follow.
          ]4

EDIT: A winning knight underpromotion I found. All other promotions lead to wins, but underpromoting to a knight is the fastest mate (mate in 1).

enter image description here

EDIT: Thanks to RosieF for catching a huge error in the bishop part.

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  • $\begingroup$ Is it possible with only 4 pieces? $\endgroup$
    – sunfishho
    Commented Jul 12, 2019 at 1:12
  • $\begingroup$ By the way, wanted to say that was a great puzzle, especially the bishop mate :) @RewanDemontay $\endgroup$
    – sunfishho
    Commented Jul 12, 2019 at 2:57
  • $\begingroup$ "2 knights cannot mate a king by themselves if the opposing side has waiting moves." Isn't it the reverse? I understood that 2 knights cannot force mate by themselves (unless they can mate in 1), but in KNN v KP the knights win (unless the P can safely queen & start checking) exactly because the defender has waiting-moves with the P which prevent a stalemate defence. $\endgroup$
    – Rosie F
    Commented Jul 12, 2019 at 5:34
  • $\begingroup$ @RosieF whoops thanks for catching that error! $\endgroup$
    – sunfishho
    Commented Jul 12, 2019 at 5:45
  • $\begingroup$ @RewanDemontay For a knight winning position, one can add a white rook on a5 to im_so_meta_even_this_acronym 's original position (so 5 pieces in total). Then 1.f8=N will be the only winning move (for the very same reason, since white rook has no checks), while other moves do not even give a draw! For the rook winning position, 1. f8=R is definitely the best move (it gives mate in 2), but it's unfortunately not the only one which wins (since 1.Ke7 or 1.Ke6 followed by 2.f8=Q also wins). $\endgroup$
    – trolley813
    Commented Jul 12, 2019 at 7:41
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Here's a situation where promoting to queen is the only winning move:

8/7P/8/8/8/2K5/kp6/8

1 h8=Q! wins

in 7: 1 ... b1=N+ 2 Kd3 (Kb4 is as good) Kb3 3 Qb8+. Or 2 ... Ka3 3 Qb8 Ka2 4 Qb4 Na3 5 Kd2 Ka1 6 Kc1 and wins next move.

1 h8=R? only draws:

1 ... b1=N+! 2 Kc2 Na3+! 3 Kc3 Nb1+! 4 Kc2 and draw by perpetual.

Here's a situation where promoting to bishop is best:

8/KNk1P

Promoting to bishop

wins in 6: 1 e8=B Kc8 2 Kb6 Kb8 3 Bd7 Ka8, forcing Black's king into the corner where White's bishop and knight can mate. 4 Nc5 Kb8 5 Na6+ Ka8 6 Bc6#.

Promoting to queen

stalemates immediately.

Promoting to knight

gives insufficient material (two knights versus bare king).

Promoting to rook

wins, but is not best, because the quickest win is in 8, e.g. 1 e8=R Kd7 2 Re5 Kc6 3 Ka6 Kc7 4 Rd5 Kc6 5 Rd4 Kc7 6 Rd6 Kc8 7 Kb6 Kb8 8 Rd8#.

As luck would have it, this morning, YouTube recommended to me a video discussing a win-study by Selezniev which contains this position:

rk/pP1p/K2p/3P w

in which, according to the video, promoting to bishop is the only way to win.

Here's a position (must surely be minimal) in which promotion to knight is the unique quickest win:

8/P1P/8/k/2K

This quickest win is

1 c8=N K~ 2 a8=Q

There are other winning moves, but the checkmate takes two or more moves longer.

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  • $\begingroup$ That's really nice! For some reason I only was thinking about scenarios with the black king in or near the corner. $\endgroup$
    – sunfishho
    Commented Jul 12, 2019 at 5:32

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