The longest checkmate question

[Uri Blass]

stockfish even see more than +7 pawns at move 56 of white of course thanks to earlier backward analysis but the longest line that it analyzed not in backward analysis was at move 68 of white and not for a long time.

[fen]8/p2N3n/p3Q1n1/p1p2rpb/6kr/2P4p/1KPP3B/8 w - - 23 56[/fen]

Stockfish_22012908_x64_avx2:
NNUE evaluation using nn-ac07bd334b62.nnue enabled

35/70 01:57 826,371k 7,048k +7.23 c3-c4 a5-a4 c2-c3 a6-a5 Kb2-a3 Kg4-f3 Qe6-e3+ Kf3-g2 Qe3-g1+ Kg2-f3 Qg1-f1+ Kf3-g4 Qf1-e2+ Rf5-f3 Qe2-e6+ Rf3-f5 Ka3xa4 Kg4-f3 Qe6-e3+ Kf3-g2 Qe3-g1+ Kg2-f3 Qg1-f1+ Kf3-e4 d2-d3+ Ke4-e3 Qf1xf5 Ng6-f4 Qf5xc5+ Ke3-e2 Bh2-g3 Nf4xd3 Qc5-e7+ Ke2-f1 Bg3xh4 g5xh4 Qe7xh4 Bh5-d1+ Ka4xa5 Nh7-g5 c4-c5 Nd3-f2 c5-c6 Ng5-f3 Qh4-h8 h3-h2 c6-c7 h2-h1Q Qh8xh1+ Nf2xh1 c3-c4 Nf3-d2 c7-c8Q Bd1-g4 Qc8-f8+ Nh1-f2 Nd7-e5 Kf1-e2 c4-c5 Nf2-d3 Ne5xd3 Nd2-b3+ Ka5-b4

36/77 03:18 1,368,861k 6,894k +7.32 c3-c4 a5-a4 c2-c3 a6-a5 Kb2-a3 Kg4-f3 Qe6-e3+ Kf3-g2 Qe3-g1+ Kg2-f3 Qg1-f1+ Kf3-g4 Qf1-e2+ Rf5-f3 Qe2-e6+ Rf3-f5 Ka3xa4 Kg4-f3 Qe6-e3+ Kf3-g2 Qe3-g1+ Kg2-f3 Qg1-f1+ Kf3-e4 d2-d3+ Ke4-e3 Qf1xf5 Ng6-f4 Qf5xc5+ Ke3-e2 Bh2-g3 Nf4xd3 Qc5-e7+ Ke2-f1 Bg3xh4 g5xh4 Qe7xh4 Bh5-d1+ Ka4xa5 Nh7-g5 c4-c5 Nd3-f2 c5-c6 Ng5-f3 Qh4-h8 h3-h2 c6-c7 h2-h1Q Qh8xh1+ Nf2xh1 c3-c4 Nh1-f2 c7-c8Q Nf3-d4 c4-c5 Bd1-f3 Qc8-a6+ Kf1-g2 Nd7-e5 Kg2-g3 Qa6-g6+ Kg3-f4 Qg6-f6+ Kf4-e3 Ne5xf3 Nd4-b3+ Ka5-b5 Nf2-e4 Qf6-b2 Ne4xc5 Qb2-c3+ Ke3-e4 Nf3-g5+ Ke4-d5

37/85 06:38 2,759,790k 6,923k +7.73 c3-c4 a5-a4 c2-c3 a6-a5 Kb2-a3 a7-a6 Ka3xa4 Kg4-f3 Qe6-e3+ Kf3-g2 Qe3-g1+ Kg2-f3 Qg1-f1+ Kf3-e4 d2-d3+ Ke4-e3 Qf1xf5 Ng6-f4 Qf5xc5+ Ke3-e2 Bh2-g3 Nf4xd3 Qc5-e7+ Ke2-f1 Bg3xh4 g5xh4 Qe7xh4 Bh5-d1+ Ka4xa5 Nh7-g5 c4-c5 Bd1-f3 Qh4-g3 Kf1-e2 Ka5-b6 Nd3-b2 Qg3-h4 Ng5-e4 Qh4xh3 Nb2-a4+ Kb6-a7 Ne4xc5 Qh3-h2+ Ke2-f1 Nd7-e5 Bf3-d1 Qh2-g3 Bd1-e2 Qg3-f4+ Kf1-e1

[Hobacle]

They claim the following is a long mate with a unique solution but I suspect the solution is wrong and there is a shorter mate in less than 226 moves.

[fen]1N2Q3/p6n/p5n1/p1p2rpb/p4k1r/K1P4p/2PP4/6B1 w - - 6 1 [/fen]

They give 56.Kc1 in the solution but 56.c4 may be stronger.
[fen]8/p2N3n/p3Q1n1/p1p2rpb/6kr/2P4p/1KPP3B/8 w - - 23 56 [/fen]

after 56.c4 black has only 2 options not to lose by a fast checkmate:
56...a4 and 56...Kf3

After 56.c4 a4 57.c3 or 56.c4 Kf3 57.Qe3+ I could get by stockfish to see more than +6 by some backward analysis of not many moves so it seems that the solution is not unique and probably shorter than 226 moves even if stockfish did not see a mate score.

It seems you were right!
This position was the version by somebody in 2008.
The original version: http://en.chessbase.com/post/longest-du ... te-problem

[Hobacle]

This position was the version by somebody in 2008.
The original version: http://en.chessbase.com/post/longest-du ... te-problem

I was wrong... Not 2008 .