Does anyone know of any problems in which a KNN-K mate is forced in the end? It is fine if you make one yourself.

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    You did almost everything correct, except that there must be a blank link line between the FEN tag and the move list. – Glorfindel Mar 26 '19 at 10:56

Here's a #4 where White has just KNN and there are no Black units blocking Black king flights:

[Title "C. Barton; Family Herald 5 Nov 1859, no. 65"]
[StartFlipped "0"]
[fen "8/8/8/8/8/6K1/4p1N1/4N1k1 w - - 0 1"]

1.Ne3 Kh1 2.Ng4 Kg1 3.Nf3+ Kh1 (3... Kf1 4.Ne3#/Nh2#) 4.Nf2#

Here's a #5 where White has just KNN and the play is a bit more interesting than in Barton's #4:

[Title "Otto Dehler; Denken und Raten 31 Dec 1934"]
[StartFlipped "0"]
[fen "8/1p6/8/8/3K4/N7/8/k1N5 w - - 0 1"]

1.Kc3 b6 (1...b5 2.Kc2 b4 3.Kb3 bxa3 4.Kc2 a2 5.Nb3#) 2.Kc2 b5 3.Nb1 b4 4.Nd2 b3+ 5.Ndxb3#
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To get the following position as a checkmate, the last move must be Nb3#.

[fen "8/8/8/8/8/1NN5/2K5/k7 b - - 0 2"]

Prior to Nb3#, black must have made a non-king move, so must have moved something to b3. If it were anything other than a pawn, it could choose to avoid the b3 square, so it's not a forced win for white. If it were a pawn on b4, it could capture the c3 knight, which is also not a forced win. Thus black must have had a pawn on c4, and be forced to capture on b3 on black's previous move.

So here's one such puzzle (LiChess):

[fen "8/8/8/8/2pN4/2N5/1PK5/k7 w - - 0 1"]

White to move and mate in 2.

It's possible to move black's pawn back a bit, and give a slightly longer puzzle with the same idea:

[fen "8/8/2p5/8/7N/2N5/1PK5/k7 w - - 0 1"]

White to move and mate in 4.

I haven't exhaustively checked all possible flips and rotations of the board (which makes more difference than I initially thought). There's also a few possible mating patterns, but I expect the idea is going to be much the same in each case.

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    "If it were anything other than a pawn, it could choose to avoid the b3 square,..." Is not quite correct. For instance (one of many counter examples): white Kc2, Nb3, Nc3, Nd4, black Ka1, Rb5. – user1583209 Mar 26 '19 at 15:44
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    Your #4 is cooked: 1 b3, 1 Nf3 and 1 Nf5 all work. The play after 1 b3 and 1 Nf3 has further duals, but that after 1 Nf5 is sound. – Rosie F Mar 28 '19 at 14:27


This is a real puzzle solving interface for one of the mate in 4 ideas given in another answer. There are many alternative solutions ending in 4.Nb3#

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