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In the pawns-only variant of chess, each side starts with 8 pawns and a king on the usual squares, no pieces, with other rules being the same as in regular chess, including promotion.

While general consensus is that chess is a theoretical draw, with best play by both sides, this is not proved because of its complexity.

The pawns-only variant looks more amenable to computer analysis.

In the pawns-only variant, is it possible for Black to imitate White or otherwise create a blockade? Is the pawns-only variant a theoretical draw?

  • 4
    Not directly relevant, but Euwe showed that the pawn-only version of the exchange Ruy Lopez is a forced win for white. en.m.wikipedia.org/wiki/Ruy_Lopez,_Exchange_Variation – Noah Snyder Feb 22 '14 at 23:51
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    How is this pawns-only variant won, do you get a queen and play on as normal on promotion, or does the first to promote win? – RemcoGerlich Mar 3 '14 at 11:18
  • @Aravind any more comments on this so far? Cheers. – Rauan Sagit Mar 19 '14 at 13:38
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    @Rauan Sagit, no more info on this. I am not accepting your answer because I am looking for a "proof", not an intuitive guess. – Aravind Apr 3 '14 at 16:55
  • @RemcoGerlich actually, it seems that the first to promote to Queen wins the game, see this link chessvariants.org/diffsetup.dir/pawnsonly.html. In fact, the pawns only variant described on that page excludes Kings, too. – Rauan Sagit Apr 3 '14 at 22:30
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I setup the position using this FEN String

4k3/pppppppp/8/8/8/8/PPPPPPPP/4K3 w - - 0 1

Using a grid engine cluster with 24 nodes, each node having 16 cores at 3.2 Ghz with 60gb of ram

The engine used was Houdini 4 pro (allowing multiple cores)

After analyzing for several days ( 4 days 12 hours 2 minutes and 15 seconds to be precise) the engine scored the position 0.00 flat with the suggested move line

1.d4 d5 2.Kd2 Kd7 3.Kd3 Kd6

Of course this is not nearly a bruteforce method, but i believe it is safe to say this is a draw. Of course @Rauan Sagit is correct. Correct play comes with intimate knowledge of pawn endings and will be even harder to achieve correct play against a skilled opponent

  • That's some powerful computer analysis! – Aravind Jun 19 '14 at 8:31
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EDIT: Now that I look closely, I have solved a different variant than what was asked. Probably this is interesting anyway :) The variant's rules are described here: http://www.chesscorner.com/tutorial/basic/pawngame/pawngame.htm

I have written a piece of software to solve this. Contrary to my and other people's expectations, and barring bugs, it shows that the game is actually a win for white. Without en passant it was a draw.

The only winning opening moves for white are 1. b4 and 1. c4 (and the symmetric 1. f4 and 1. g4).

Curiously, all other white opening moves are a win for black. 1. d4 is refuted by 1. ... b5 0-1; all other black responses are wins for white.

I put the code here: https://github.com/sliedes/pawnsonly. With the default setup it uses 25 gigabytes of memory for the transposition table and I seem to remember it takes a few hours to solve the 8x8 case. I have not tested how slow it is with less memory.

Here are some results of the first two or three moves:

Some of the lines end with "1/2-1/2+" or "1/2-1/2-". "1/2-1/2+" means that white can force at least a draw, and no further search was done because this information was enough for perfect play from the starting position (so it might also be a white win). Correspondingly, "1/2-1/2-" means that black is able to force at least a draw.

  • Great work! Do you have some sample games? What's the key idea for W's breakthrough? What was the depth of the search? – Aravind Apr 5 '16 at 9:18
  • I don't currently have any sample games. I've been thinking about how easy it would be to make it somehow queriable; the transposition table could presumably be used to speed up queries. The search was exhaustive, i.e. it continued until the game was over, and therefore the result is exact (barring bugs). It would be interesting to know how many moves it takes for white to win, but unfortunately even that is nontrivial with alpha-beta and the very compact transposition table I use. – Sami Liedes Apr 5 '16 at 22:26
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Yes, my guess would be that the pawn-only chess variant is a draw with exact play from both sides. Yet, it is of course possible to win for either side, as long as you know what plans to choose between. Since the kings are the only pieces left on the board (apart from the pawns), they will play a key role. The goal will be to create weaknesses in the enemy pawn structure and invade them with the king. Also, to have a good understanding for which pawn endings that are drawn and which ones that are won (e.g. the distanced passed pawn). So while it is objectively drawn, it is still hard to play over the board against a skilled opponent.

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