12 December 2016

TCEC Season 9 Superfinal Followup

In last week's post, TCEC Season 9 Superfinal Week 4, I wrote,

Congratulations to the Stockfish team on becoming the defacto, unofficial World Computer Chess Champion.

Now that the event is over, what can be said about the games? I'll leave annotations of complete games to the experts, because I don't know how to start understanding engines that routinely calculate variations to depths of 40 ply. Instead I'll look at the openings where White won both games (all decisive games were won by White) like I did last year in Battering the Gruenfeld (March 2015; TCEC Season 7). The game pairs were:-

  • g.17/18 Sicilian: Dragon, Yugoslav, Main Line, 12.h4 Nc4: 14.h5
  • g.65/66 Slav: Smyslov, 6.e4 Bg4 7.Bxc4 e6
  • g.71/72 French: Classical, Steinitz, 9.Qd2 Nc6 10.dxc5 Qxc5

As documented in TCEC Season 9 Superfinal Week 2, the first game in the list (g.17) has already received attention for the way it was scored.

A Stockfish win in game 17 provoked a genuine controversy. The final position of Queen vs. two Bishops [Q:2B] is shown in the following diagram, Black to move.

The position is shown below in the diagram on the left, which is a tablebase win for White. Instead of continuing a discussion of the controversy, I would rather focus on the position itself. I had always thought that Q:2B was generally a draw, because the two Bishops should be able to prevent the enemy King from getting too close. The diagram on the right shows position 605 (attributed to Lolli, 1763; flipped on the horizontal axis) from Fine's 'Basic Chess Endings', which is a draw. Why is the first diagram a win and the second a draw?

I did some simple manipulations of both positions, moving all pieces up/down a rank or left/right a file. If you move the left position down a rank, so that the Black pieces are on the same squares as in the right position, the game is a draw. If you move the right position up a rank, the game is a win for White. The squares of the White pieces don't seem to matter much, as long as there are no immediate tactics.

If you move either position up/down or left/right an arbitrary number of ranks/files, the position is always a win for White. From this I concluded that the position in BCE 605 forcing a draw is a special case. Black defends successfully because the White King can't get 'behind' the huddled Black pieces, as it can in the position on the left.

You might think it would be a simple matter to go from the left position to the right position by regrouping the Black pieces, especially since the White King is so far away. It turns out that the White Queen can singlehandedly prevent any regrouping. All that Black can do is mark time. When the White King gets close, one of the Bishops is inevitably lost.

I also looked at the final positions of other games that ended by reaching a 'TB position', i.e. terminated by a five-piece tablebase. All of them were draws and none of them was particularly interesting. Most were Bishops of opposite color or Rook against two Pawns, where any good player can foresee the result. More interesting are those three pairs of busted(?) openings that I flagged earlier. I'll tackle those in another post.

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