A Position From Hard Talkches 2020 #34

[acase]

This is actually an endgame study by former World Chess Champion Vassily Smyslov that used to be very difficult for engines, however ShashChess now sees a mate in 28 in this position:

[pgn][Event "Mate in 28"] [Site "Columbus, Ohio"] [Date "2023.08.02"] [Round "?"] [White "?"] [Black "?"] [Result "1-0"] [Time "19:48:47"] [TimeControl "0+5"] [SetUp "1"] [FEN "2b1rk2/5p2/p1P5/2p2P2/2p5/7B/P7/2KR4 w - - 0 1"] [Termination "normal"] [PlyCount "55"] [WhiteType "program"] [BlackType "program"] [Comment "bm f6; id \"Hard-Talkchess-2020.034\" ;"] 1. f6 Bxh3 (1. .. c3 2. c7 Bxh3 3. Rd8 Bf5 4. Kd1 c2+ 5. Kc1 c4 6. a4 a5 7. Kd2 c1=R 8. Kxc1 c3 9. Kd1 c2+ 10. Kc1 Bg4 11. Kxc2 Bf5+ 12. Kc3 Bh3 13. Kc4 Bf1+ 14. Kc5 Bh3 15. Kb5 Kg8 16. Rxe8+ Kh7 17. Kxa5 Kg6 18. Kb6 Kxf6 19. a5 Kg7 20. a6 Kg6 21. a7 Bc8 22. a8=Q Kg5 23. Qxc8 f5 24. Qd7 Kf4 25. c8=Q Kf3 26. Qxf5+ Kg3 27. Rg8+ Kh4 28. Qh3#) (1. .. a5 2. c7 Bxh3 3. Rd8 a4 4. Kb2 Be6 5. Kc2 Bc8 6. Kc3 Be6 7. a3 Bc8 8. Kxc4 Bg4 9. Kxc5 Bf5 10. Kb4 Kg8 11. Rxe8+ Kh7 12. Kxa4 Kg6 13. Kb4 Bd7 14. c8=Q Bxc8 15. Rxc8 Kf5 16. a4 Ke6 17. Re8+ Kxf6 18. a5 Kg5 19. a6 f6 20. Rg8+ Kf4 21. a7 Ke3 22. a8=Q f5 23. Kc3 f4 24. Qe8+ Kf2 25. Kd2 f3 26. Qe1#) 2. c7 Bf5 (2. .. Kg8 3. Rd8 Rf8 4. Kc2 c3 5. Kxc3 Be6 6. a3 a5 7. a4 c4 8. Kd4 c3 9. Kxc3 Bg4 10. Kc4 Be6+ 11. Kb5 Kh7 12. Rxf8 Kg6 13. Kxa5 Kxf6 14. c8=Q Bxc8 15. Rxc8 Kg7 16. Kb6 f5 17. a5 f4 18. a6 f3 19. Rc4 Kf6 20. a7 f2 21. a8=Q f1=Q 22. Qf8+ Kg6 23. Qxf1 Kg5 24. Qf3 Kh6 25. Rg4 Kh7 26. Qh1#) (2. .. c3 3. Rd8 Bf5 4. Kd1 c2+ 5. Kc1 c4 6. a4 a5 7. Kd2 c1=R 8. Kxc1 c3 9. Kd1 c2+ 10. Kc1 Bg4 11. Kxc2 Bf5+ 12. Kc3 Bh3 13. Kc4 Bf1+ 14. Kc5 Bh3 15. Kb5 Kg8 16. Rxe8+ Kh7 17. Kxa5 Kg6 18. Kb6 Kxf6 19. a5 Kg7 20. a6 Kg6 21. a7 Bc8 22. a8=Q Kg5 23. Qxc8 f5 24. Qd7 Kf4 25. c8=Q Kf3 26. Qxf5+ Kg3 27. Rg8+ Kh4 28. Qh3#) 3. Rd8 c3 4. Kd1 c2+ 5. Kc1 c4 6. a4 a5 7. Kb2 c1=Q+ 8. Kxc1 c3 9. Kd1 c2+ 10. Kc1 Be6 11. Kxc2 Bc8 12. Kc3 Ba6 13. Kd4 Bc8 14. Kc5 Bf5 15. Kb6 Kg8 16. Rxe8+ Kh7 17. Kxa5 Kg6 18. Kb6 Kxf6 19. a5 Kg6 20. a6 Bd7 21. a7 f5 22. a8=Q Bc8 23. Qxc8 Kg5 24. Qa8 Kf4 25. c8=Q Kg3 26. Qxf5 Kh2 27. Qf2+ Kh3 28. Qaf3# 1-0[/pgn]

Are there any shorter mates spotted by engines?

[peter]

This is actually an endgame study by former World Chess Champion Vassily Smyslov that used to be very difficult for engines, however ShashChess now sees a mate in 28 in this position:

[pgn][Event "Mate in 28"] [Site "Columbus, Ohio"] [Date "2023.08.02"] [Round "?"] [White "?"] [Black "?"] [Result "1-0"] [Time "19:48:47"] [TimeControl "0+5"] [SetUp "1"] [FEN "2b1rk2/5p2/p1P5/2p2P2/2p5/7B/P7/2KR4 w - - 0 1"] [Termination "normal"] [PlyCount "55"] [WhiteType "program"] [BlackType "program"] [Comment "bm f6; id \"Hard-Talkchess-2020.034\" ;"] 1. f6 Bxh3 (1. .. c3 2. c7 Bxh3 3. Rd8 Bf5 4. Kd1 c2+ 5. Kc1 c4 6. a4 a5 7. Kd2 c1=R 8. Kxc1 c3 9. Kd1 c2+ 10. Kc1 Bg4 11. Kxc2 Bf5+ 12. Kc3 Bh3 13. Kc4 Bf1+ 14. Kc5 Bh3 15. Kb5 Kg8 16. Rxe8+ Kh7 17. Kxa5 Kg6 18. Kb6 Kxf6 19. a5 Kg7 20. a6 Kg6 21. a7 Bc8 22. a8=Q Kg5 23. Qxc8 f5 24. Qd7 Kf4 25. c8=Q Kf3 26. Qxf5+ Kg3 27. Rg8+ Kh4 28. Qh3#) (1. .. a5 2. c7 Bxh3 3. Rd8 a4 4. Kb2 Be6 5. Kc2 Bc8 6. Kc3 Be6 7. a3 Bc8 8. Kxc4 Bg4 9. Kxc5 Bf5 10. Kb4 Kg8 11. Rxe8+ Kh7 12. Kxa4 Kg6 13. Kb4 Bd7 14. c8=Q Bxc8 15. Rxc8 Kf5 16. a4 Ke6 17. Re8+ Kxf6 18. a5 Kg5 19. a6 f6 20. Rg8+ Kf4 21. a7 Ke3 22. a8=Q f5 23. Kc3 f4 24. Qe8+ Kf2 25. Kd2 f3 26. Qe1#) 2. c7 Bf5 (2. .. Kg8 3. Rd8 Rf8 4. Kc2 c3 5. Kxc3 Be6 6. a3 a5 7. a4 c4 8. Kd4 c3 9. Kxc3 Bg4 10. Kc4 Be6+ 11. Kb5 Kh7 12. Rxf8 Kg6 13. Kxa5 Kxf6 14. c8=Q Bxc8 15. Rxc8 Kg7 16. Kb6 f5 17. a5 f4 18. a6 f3 19. Rc4 Kf6 20. a7 f2 21. a8=Q f1=Q 22. Qf8+ Kg6 23. Qxf1 Kg5 24. Qf3 Kh6 25. Rg4 Kh7 26. Qh1#) (2. .. c3 3. Rd8 Bf5 4. Kd1 c2+ 5. Kc1 c4 6. a4 a5 7. Kd2 c1=R 8. Kxc1 c3 9. Kd1 c2+ 10. Kc1 Bg4 11. Kxc2 Bf5+ 12. Kc3 Bh3 13. Kc4 Bf1+ 14. Kc5 Bh3 15. Kb5 Kg8 16. Rxe8+ Kh7 17. Kxa5 Kg6 18. Kb6 Kxf6 19. a5 Kg7 20. a6 Kg6 21. a7 Bc8 22. a8=Q Kg5 23. Qxc8 f5 24. Qd7 Kf4 25. c8=Q Kf3 26. Qxf5+ Kg3 27. Rg8+ Kh4 28. Qh3#) 3. Rd8 c3 4. Kd1 c2+ 5. Kc1 c4 6. a4 a5 7. Kb2 c1=Q+ 8. Kxc1 c3 9. Kd1 c2+ 10. Kc1 Be6 11. Kxc2 Bc8 12. Kc3 Ba6 13. Kd4 Bc8 14. Kc5 Bf5 15. Kb6 Kg8 16. Rxe8+ Kh7 17. Kxa5 Kg6 18. Kb6 Kxf6 19. a5 Kg6 20. a6 Bd7 21. a7 f5 22. a8=Q Bc8 23. Qxc8 Kg5 24. Qa8 Kf4 25. c8=Q Kg3 26. Qxf5 Kh2 27. Qf2+ Kh3 28. Qaf3# 1-0[/pgn]

Are there any shorter mates spotted by engines?

From the original study the first 3 moves are cut off.
Here's what HHdbVI gives:

[pgn][Event "64"] [Site "?"] [Date "1938.??.??"] [Round "?"] [White "Smyslov=V"] [Black "(+0440.45c1f8)"] [Result "1-0"] [SetUp "1"] [FEN "1r3k2/5p2/p1P1b1p1/2p5/2p1PP2/8/P7/2KR1B2 w - - 0 1"] [PlyCount "19"] [EventDate "1938.??.??"] {(EG#07677).} 1. f5 $1 (1. c7 $2 Rc8 2. Rd8+ Ke7) (1. Bxc4 $2 Bxc4 2. c7 Re8 3. f5 (3. Rd8 Be6) 3... a5) 1... gxf5 2. Bh3 Re8 (2... Ke7 3. exf5 Bc8 4. f6+ Kxf6 5. Rd8) (2... Rc8 3. exf5) 3. exf5 (3. c7 $2 Bc8 4. exf5 (4. Bxf5 Ke7 $1) 4... f6 $1) 3... Bc8 4. f6 $1 (4. Rd7 $2 Re3 $1 (4... Bxd7 $2 5. cxd7 Rd8 6. f6 Kg8 7. Bf5 $1) (4... Kg7 $2 5. c7 $1 Re1+ (5... Bxd7 6. f6+ Kxf6 7. Bxd7) 6. Kd2 Rh1 7. Bg4 $1 (7. Rd8 $2 c3+ 8. Kc2 Rh2+) (7. f6+ $2 Kg6 $1 8. Bg4 Rh4 9. Rd8 Bxg4 10. Rg8+ Kxf6 11. Rxg4 Rh8) 7... Rg1 (7... Rh4 8. Rd8) 8. f6+ $1 Kxf6 (8... Kg6 9. Rd8 Bxg4 10. Rg8+ Kxf6 11. Rxg4) 9. Rxf7+ Kxf7 10. Bxc8) (4... f6 $2 5. Rc7 a5 6. a4) 5. Rc7 (5. f6 Ke8 $1) 5... Rxh3 6. Rxc8+ Ke7) 4... Bxh3 5. c7 a5 (5... c3 {<main>} 6. Rd8 $1 Bf5 7. a4 a5 8. Kd1 c2+ 9. Kc1 c4 10. Kd2 c3+ 11. Kc1) (5... Bf5 6. Rd8 c3 7. Kd1 a5 8. a4 $1 c2+ 9. Kc1 $1 c4 10. Kd2) 6. Rd8 $1 (6. a4 $2 Kg8 $1 7. Rd8 Rf8 8. Kc2 Bg4 9. Kc3 Be6) (6. Kb2 $2 Re2+ 7. Kc3 Kg8) 6... a4 7. Kb2 $1 (7. Kc2 $2 a3) 7... Be6 (7... Bf5 8. Kc3 Be6 9. a3) 8. Kc2 $1 (8. Ka3 $2 c3) (8. Kc3 $2 a3) 8... Bf5+ (8... a3 9. Kc3) 9. Kc3 Be6 10. a3 1-0 [/pgn]

With this one starting position YACPDB has it too:

https://www.yacpdb.org/#search/MXIzazI1 ... LzEvMA==/1

And yes, the DTM starting with 4.(1.) f6 seems to be right just as for a quick Backward with Huntsman:

2b1rk2/5p2/p1P5/2p2P2/2p5/7B/P7/2KR4 w - - 0 1

Analysis by The Huntsman 1:

4.f6 Lxh3 5.c7 Lf5 6.Td8 c3 7.Kd1 c2+ 8.Kc1 c4 9.a4 a5 10.Kb2 c1D+ 11.Kxc1 c3 12.Kd1 c2+ 13.Kc1 Le6 14.Kxc2 Lf5+ 15.Kc3 Le6 16.Kd4 Lh3 17.Kc5 Lf5 18.Kb5 Kg8 19.Txe8+ Kh7 20.Kxa5 Kg6 21.Kb4 Kxf6 22.a5 Kg6 23.a6 Le6 24.Txe6+ Kf5 25.c8D fxe6 26.Df8+ Kg4 27.a7 e5 28.a8D e4 29.Dxe4+ Kh3 30.Dh8+ Kg3 31.Dhh4#
+- (#28) Tiefe: 77/56 00:01:10 4740MN