user15543: 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.

user3456: https://www.apronus.com/chess/puzzle/?p=08X8X2p5X8X7NX2N5X1PK5Xk7_w_-_-_0_1&N=4&w=rqsqpqq-aqfqrqw-aqfqiqw-aqpqiqq-aqpqsqw-iqrqpqq-iqoqiqq-iqoqsqw&h=White%20to%20move%20and%20win%20by%20checkmate%20in%204%20moves&m=9
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#

user3456: The mate in 4 from another answer can be presented as an interactive puzzle with this link
https://www.apronus.com/chess/puzzle/?p=08X8X8X8X8X6K1X4p1N1X4N1k1_w_-_-_0_1&N=4&w=uqtqowd-uqtqoqp-uqtqoqf&h=%3Ch1%3EMate%20in%204%3C%2Fh1%3EC.%20Barton%3B%20Family%20Herald%205%20Nov%201859%2C%20no.%2065&m=9
While the mate in 5 is this https://www.apronus.com/chess/puzzle/?p=08X1p6X8X8X3K4XN7X8Xk1N5_w_-_-_0_1&N=0&w=owaqeqiqw-oqsqaqoqw&h=%3Ch1%3EMate%20in%205%3C%2Fh1%3EOtto%20Dehler%3B%20Denken%20und%20Raten%2031%20Dec%201934&m=9

user10380: 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#


user2785: 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.

15543: <p>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.</p>
 3456: <p><a href="https://www.apronus.com/chess/puzzle/?p=08X8X2p5X8X7NX2N5X1PK5Xk7_w_-_-_0_1&amp;N=4&amp;w=rqsqpqq-aqfqrqw-aqfqiqw-aqpqiqq-aqpqsqw-iqrqpqq-iqoqiqq-iqoqsqw&amp;h=White%20to%20move%20and%20win%20by%20checkmate%20in%204%20moves&amp;m=9" rel="nofollow noreferrer">https://www.apronus.com/chess/puzzle/?p=08X8X2p5X8X7NX2N5X1PK5Xk7_w_-_-_0_1&amp;N=4&amp;w=rqsqpqq-aqfqrqw-aqfqiqw-aqpqiqq-aqpqsqw-iqrqpqq-iqoqiqq-iqoqsqw&amp;h=White%20to%20move%20and%20win%20by%20checkmate%20in%204%20moves&amp;m=9</a></p>

<p>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#</p>
 3456: <p>The mate in 4 from another answer can be presented as an interactive puzzle with this link
<a href="https://www.apronus.com/chess/puzzle/?p=08X8X8X8X8X6K1X4p1N1X4N1k1_w_-_-_0_1&amp;N=4&amp;w=uqtqowd-uqtqoqp-uqtqoqf&amp;h=%3Ch1%3EMate%20in%204%3C%2Fh1%3EC.%20Barton%3B%20Family%20Herald%205%20Nov%201859%2C%20no.%2065&amp;m=9" rel="nofollow noreferrer">https://www.apronus.com/chess/puzzle/?p=08X8X8X8X8X6K1X4p1N1X4N1k1_w_-_-_0_1&amp;N=4&amp;w=uqtqowd-uqtqoqp-uqtqoqf&amp;h=%3Ch1%3EMate%20in%204%3C%2Fh1%3EC.%20Barton%3B%20Family%20Herald%205%20Nov%201859%2C%20no.%2065&amp;m=9</a></p>

<p>While the mate in 5 is this <a href="https://www.apronus.com/chess/puzzle/?p=08X1p6X8X8X3K4XN7X8Xk1N5_w_-_-_0_1&amp;N=0&amp;w=owaqeqiqw-oqsqaqoqw&amp;h=%3Ch1%3EMate%20in%205%3C%2Fh1%3EOtto%20Dehler%3B%20Denken%20und%20Raten%2031%20Dec%201934&amp;m=9" rel="nofollow noreferrer">https://www.apronus.com/chess/puzzle/?p=08X1p6X8X8X3K4XN7X8Xk1N5_w_-_-_0_1&amp;N=0&amp;w=owaqeqiqw-oqsqaqoqw&amp;h=%3Ch1%3EMate%20in%205%3C%2Fh1%3EOtto%20Dehler%3B%20Denken%20und%20Raten%2031%20Dec%201934&amp;m=9</a></p>
 10380: <p>Here's a #4 where White has just KNN and there are no Black units blocking Black king flights:</p>

<pre><code>[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#
</code></pre>

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

<pre><code>[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#
</code></pre>
 2785: <p>To get the following position as a checkmate, the last move must be Nb3#.</p>

<pre><code>[fen "8/8/8/8/8/1NN5/2K5/k7 b - - 0 2"]
</code></pre>

<p>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.</p>

<p>So here's one such puzzle (<a href="https://lichess.org/analysis/standard/8/8/8/8/2pN4/2N5/1PK5/k7_w_-_-" rel="noreferrer">LiChess</a>):</p>

<pre><code>[fen "8/8/8/8/2pN4/2N5/1PK5/k7 w - - 0 1"]
</code></pre>

<p>White to move and mate in 2.</p>

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

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

<p>White to move and mate in 4.</p>

<p>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.</p>
