Senket — Invading the Corner

 

Red should secure at A.

Red should secure at A.

(If you haven’t read them already, check out the rules for Senket)

If a player makes too large a territory in the corner without protecting it, his opponent can take advantage by invading and making a territory of his own inside. This is possible with any territory, but especially so since it takes only two posts to make a (very small) territory in the corner. In the diagram, Red has a nice territory of 14 units, worth 196 points, but he’s too far away from the corner, and Blue will make him pay for this mistake. Note that the move shown at A would lock this territory up for Red; any move by Blue at that point would only result in prisoners for Red.

 

If Red does nothing, Blue can make territory immediately at B.

If Red does nothing, Blue can make territory immediately at B.

In this diagram, Blue has made his first move in punishing Red. If Red doesn’t respond, Blue can make territory in one more move, as shown at B. It’s only 1 unit of territory for Blue, but it costs Red his 14 units of territory, so this move is worth 197 points as it stands.

Red has already lost the corner; there’s nothing he can do to stop Blue. But the situation isn’t hopeless. Here are two examples of what Red can do to salvage some territory.

 

invade3

Blue can connect at C or D. Red can't block both.

Here Red has taken the direct approach to try to thwart Blue: he placed a post on the spot Blue could connect to in order to make territory. This prevents Blue from playing there, but Blue can extend down to the right as shown. This gives Blue two paths to the side, as shown by the hashed posts and fences. There is nothing Red can do to prevent Blue from connecting one way or the other to make territory. 

Blocking to the left (by placing a post at C) makes no sense. Blue would connect at D for 4 units of territory and 2 prisoners. 

 

invade4

Red limits Blue to two units of territory, but he can do better.

Blocking to the right is better. Red plays at 4, shown here, and Blue is forced to play at 5 to make territory (otherwise Blue is just prisoners). Then Red plays at 6, connecting two ways and making territory. The result is that Red gets 8 units instead of 14, and Blue gets 2 units, so this play would be worth 136 points to Blue.

In the game of Go, Sente means (roughly) that Blue initiated the exchange, and after the sequence is complete it is Blue’s move again. This is important because Blue determines the next area of play. The opposite term is Gote, where Blue initiated the exchange, but needs makes the last move, so Red decides where to play next. In Go, if a player has two options and one is Sente while the other is Gote, the player will value the Sente move at roughly twice its actual unit value. The difference between Sente and Gote in Fences does not appear to be as great as in Go, but it’s still a consideration.

 

Red's best play. He is down 76 points on the exchange.

Red's best play. He is down 76 points on the exchange.

Red has a better play than any of the above. Instead of blocking Blue from the single unit play, Red makes the same play as was shown at the top of this section — the play that would have secured the territory in the first place if Blue didn’t already have a post inside. Blue is forced to take 1 unit of territory — it’s all that’s left. Red follows by connecting to the bottom. The result is that Red salvages 11 of his original 14 units, and Blue gets just 1 unit. Before this exchange, Red had 14 units of territory for 196 points. Now Red has 11 units for 121 points, and Blue has gained 1 unit for 1 point. So this play was worth 196 – 121 + 1 = 76 points for Blue.

 

These points tend to secure the corner.

These points tend to secure the corner.

The key factor for Red’s security is the ability to connect to the corner, or the point just above it or to the right of it, thus preventing the 1 unit territory Blue secured in the example above. If Red’s fence can connect to any of those three points, Red is generally secure in the corner. These eleven points can connect to secure the corner.

Even if Red occupies one of these points, it can be possible for Blue to leverage the possibility of a 1 unit territory to build out to the side if there is some room. Some of the example problems later will demonstrate this.

Finally, sometimes it’s not worth securing the corner. If the potential damage of an invasion is less than what moves elsewhere are worth, securing the corner can wait.

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