Marin's K+P Analysis Error & 5-Pawn Swings by Engines

[KWRegan]

I found a major hole in GM Marin's big analysis of the Adams-Polgar Corus Rd. 12 King and Pawn ending on ChessBase at http://www.chessbase.com/newsdetail.asp?newsid=4416. In this position:

8/7p/6p1/p1k2p2/1pp2P2/P1P2P1P/2P5/2K5 w - - 0 38

[fen]8/7p/6p1/p1k2p2/1pp2P2/P1P2P1P/2P5/2K5 w - - 0 38 [/fen]

Marin gave only 38.axb3, but 38.Kb2! holds. He may have missed this for 2 reasons: usually allowing an outside passer is suicide, and Fritz (my 9 & DF10 with 1024MB hash; he used 11) instantly sees this as a 2-pawn blunder and gives Black 3.20+ to depth 20 or so. When my DF10 is run a long time, it eventually flatlines to 0.00. HIARCS 11.2 and Shredder 9.1 give similar high evals, but Rybka2.2n2, 2.3.1, and 2.3.1LK give only about -1.00 all the time, while Rybka 2.3.2a gives -2.09 or thereabouts---with most of my other engines (single core) in the mid-to-high -1.nn's.

My own analysis posted on Dennis Monokroussos' blog "The Chess Mind" at http://chessmind.powerblogs.com/posts/1201988305.shtml (game link http://chessmind.powerblogs.com/files/w ... by_kwr.htm, my full PGN downloadable at http://www.cse.buffalo.edu/~regan/chess ... 08full.pgn) includes a position in which the same defensive idea causes far bigger swings by engines:

8/1k6/7p/ppp2p2/2pP1p1P/2P2P2/PKP5/8 w - - 0 51
[fen]8/1k6/7p/ppp2p2/2pP1p1P/2P2P2/PKP5/8 w - - 0 51[/fen]

I've observed DF10 give Black 4.70--5.22 past depth=30 many times, but White to move can hold by 51. dxc5 Kc6 52. a3! Kd5 53. Kb1! Kxc5 54. Kc1 Kd5 55. Kd2 Kc6 56. Kd1! Kd6 57. Kd2 Kd5 58. Kd1! Ke6 59. Ke2 Kf6 60. Kf2 Kg6 61. Kg2 Kh5 62. Kh3 a4 63.Kh2! As after 38.Kb2, White gets counterplay along the Rook's file and Black cannot win any queening races. I have "proved" this by running DF10 at 63.Kh2 until it says 0.00, then backtracking this eval to the beginning. However, HIARCS 11.2MP persists with 9.03 to Black even when I play some more moves after 63.Kh2!

I claim this as a kind of record for a drawn position that is "normal", i.e. not featuring extra material that is blockaded or subject to stalemate defenses (like one in the Dec.-Jan. "Adjudications" column in Selective Search magazine), and not with winnable Queening races or one side a Queen down but holding. What other examples are out there?

Thanks, and enjoy! ---Ken Regan

(P.S.: The positions in the preview are coming out from Black's side---I don't see how to fix at http://www.hiarcs.net/forums/viewtopic.php?t=12.)

[Steve B]

(P.S.: The positions in the preview are coming out from Black's side---I don't see how to fix at http://www.hiarcs.net/forums/viewtopic.php?t=12.)

Hi Ken
i took the liberty of editing your post to change the fen string to show White from bottom

you need the W at the end of the fen string like so:
8/7p/6p1/p1k2p2/1pp2P2/P1P2P1P/2P5/2K5 w - - 0 38

[fen]8/7p/6p1/p1k2p2/1pp2P2/P1P2P1P/2P5/2K5 w - - 0 38[/fen]

W Chess Regards
Steve

[KWRegan]

Thanks! The help thread which I linked doesn't include the w/b or - - part, and I thought maybe they were skipped because that isn't needed to *display* a diagram.

Meanwhile, I need to fix the last sentence in my first pgh after getting my laptop back from my son and checking. It should be: HIARCS 11.2 and Shredder 9.1 give similar high evals, but Rybka2.2n2, 2.3.1, and 2.3.1LK give only about -1.00 all the time, while Rybka 2.3.2a gives -2.09 or thereabouts---with most of my other engines (single core) in the mid-to-high -1.nn's.

[Harvey Williamson]

Thanks! The help thread which I linked doesn't include the w/b or - - part, and I thought maybe they were skipped because that isn't needed to *display* a diagram.

Meanwhile, I need to fix the last sentence in my first pgh after getting my laptop back from my son and checking. It should be: HIARCS 11.2 and Shredder 9.1 give similar high evals, but Rybka2.2n2, 2.3.1, and 2.3.1LK give only about -1.00 all the time, while Rybka 2.3.2a gives -2.09 or thereabouts---with most of my other engines (single core) in the mid-to-high -1.nn's.

Done - please check i got it right.

[KWRegan]

Wow, that is what we call "service"! The fixes are exactly right.

Meanwhile, is there an article or repository of "normal" positions on which engines diverge widely, or swing from 5.00+ at high depths to 0.00 without there being any hidden sacrificial tactics?