Two Squares (Life in small groups)
KjeldPetersen: With this theory I would like to start a discussion if it is possible to view life and death in go in a different way. My attempt here is a semi-set-theory-math approach, since I do not know better. I hope that this theory will be received positively and that someone with higher math skills could take over and describe it more accurately. Maybe this theory will help you; maybe you will find it rubbish. At least, please keep an open mind, and participate with constructive criticism.
What I want to do is try to see if it is possible to identify properties of different situations using the two square view. It may turn out to be just as difficult as any other method. I will start with examples that are almost filled, missing just one stone to complete the two squares. But I hope to extend the examples to including how to select moves in sparse positions so as to better ensure the formation of two squares, and hence life for your groups.
For a theory to be useful it must be give correct results when applied. So I challenge you to find counter examples for which this theory incorrectly determines the status of a group.
I hope that I, with my Two Square theory, can contribute with some knowledge that let people understanding some of the fundamental principles in the game, that otherwise could stay hidden for long time. I played Go for very long time, before I started to understand the concept of good shape. Two Squares is an idea that I have, on how to describe principles of good shape.
Learning the game of go is hard. You have to spend hundreds of hours studying and playing games to slowly bit by bit to understand more and more about the game. Learn how patterns form, which patterns are strong and which to avoid completely.
Today there are many sources you can go to gain knowledge. There are several online game servers and a ton of books you can buy. There are go clubs in nearly all major city in the world. And maybe also on every habitable planet in the universe where intelligent life has evolved.
Go is a game of patterns. The alternating nature of the game and the constant battle for territory supremacy creates patterns that keep reappearing. Some patterns occur more frequently than others. Some patterns are more effective than others. Some patterns creates life. Some patterns prevents life. Some patterns are counterproductive. In this article I will show one possible view on patterns for stones to survive on the board. The patterns that I will concentrate on are the square frame and rectangle frame as shown below.
Definition: Squares consist of 4 side stones and 4 corner stones that are placed around an empty center position.
Definition: Rectangles are like squares except they have two more side stones and one additional empty center position.
I will start explaining things with squares only. But later I will extend it to include rectangles also. The focus in the examples are on how squares and rectangles are formed and prevented. So in the examples below you should mostly focus on the square and rectangle parts of the boards. Not on the whole board. Not all examples are set up as actual game battle situations. At the start of the article very few are, but later I will show some actual game situation where the Two Squares theory can be applied. This article focuses on a theoretical approach about squares and rectangle shapes on the Go board.
Life (point-giving life – we will have a look at non-point-giving life, i.e. seki, later) exists when 2 squares are played out so they have some kind of connection between them. The 2 connected squares below mutually ensure existence (life) by preventing the opponent from occupying all liberties. If the opponent were to play on any of the free position in the center of any square then it would be illegal (in most rule sets) or captured (in a few rule sets). In both cases it can’t remain on the board to occupy a liberty.
But squares doesn’t need to be so far apart that they need a string of stones to connect them. Placing the squares adjacent or to some degree overlapping is more efficient, as it requires fewer stones. Placing the squares on the side of the board even requires even fewer stones. But I will come back to that. Below are adjacent and overlapping placed squares.
Squares don’t need to be fully formed with all eight stones at all time to be valid. Some of the corner stones can be omitted, and still ensure integrity for life. The side stones are essential, and cannot be omitted. And another thing that cannot be omitted, or rather should be omitted, is the center liberty, “the eye in the square”. This must remain unfilled at all time. The corner stones however can be omitted to some extent, but the conditions for omitting them depend on the number of side stones that have separate connections to other squares. For a square with one connection over to some other square, one corner stones can be omitted. Any one of the corner stones can be selected, but only one.
A square with two connections to two other squares or two connections to just one other square, can omit two of the corner stones. But it must follow a certain structure. Only one single corner stone between each pair of connections either way around the square can be omitted. So both clockwise and counter-clockwise between the two connections one single stone can be omitted.
With three connections from one square to some other squares (one, two or three, it doesn’t matter) three corner stones can be omitted. But still only one single corner stone between each pair of connections can be omitted.
And of cause with four connections from a square to some other squares (one, two, three or four, it doesn’t matter) all four corner stones can be omitted.
As you can see the number of corner stones that can be omitted is equal to the number of connections to other squares. It doesn’t matter if two, three or four connections runs over to the same other square. The question is just how many distinct connections to some other squares are running out of a square.
The thing about omitting corner stones is however a double-edged sword. A player may omit some corner stones, when they establish squares and connections between them, to save stones for other purposes, but their opponent can also occupy the corner positions of nearly formed squares, thereby forcing them to unwillingly establish more connections between squares. More connections mean more stones spent on non-point-giving patterns.
You may save some stones when forming two squares by overlapping them to some extend. (Overlapping two squares completely makes no sense.) In one case you can overlap the two squares by four positions, and hereby make one squares center position (eye) a corner position of the other square. And vice versa.
NB: That the center position of a square must remain empty takes priority over occupying the corner position of the other square, since it is the empty center positions of the squares that in the end ensures life for all the stones.
But this means that one corner of each square must be unoccupied. The question is now how many connections there are between the two overlapping squares, and hence how many additional corner stones can be omitted in each square. The answer in this case is that there is two connections between the two overlapping squares, since two side stones are shared among the two squares. So two corner stones in each square can be omitted. Since one corner stone in each square is already omitted by the center positions of the other square, only one addition stone can be omitted.
The edge of the board (first line) has some special properties in regard to counting connections and omitting corner stones. Squares with the unfilled centered position (eye) placed on the first line do not need the missing side and corner stones of the square to form a valid square. But as regards the number of corner stones outside the board, one should be considered already omitted. That means that two connections are needed in order to omit yet another corner stone, and three connections are needed to omit two corner stones.
All the (a) squares have only one single connection, and can therefore only omit one corner stone. But the (a) square at the edge of the board has already one corner stone omitted by the property of the edge, and therefore cannot omit any of the corner stones on the board.
The (b) squares all have two connections to other squares, so two corner stones can be omitted. But since they are located on the edge of the board, one corner stone is already counted as being omitted, and only one additional stone can be omitted.
With the (c) square there are three connections, but again as it is located on the edge of the board a maximum of two corner stones can be omitted from this square.
A special position arises when you have two squares both with two connections running over to each other. Note: two or more squares of your opponent could be settled in between your two connections. Since both squares have two connections to some other squares, two corner stones can be omitted. This forms what is believed to be a Two-Headed Dragon.
Two squares that overlap at four points, as described in the section with the center of a square, also have two squares with two connections to each other. So it could be argued that this also is a Two-Headed Dragon even though there are no opposing stones between the two connections.
On a very small 2-by-2 board two stones that are placed diagonally also forms two squares with two connections to each other. For both squares one corner stones is omitted by the center of the other square, and the another corner stone is omitted by the property of the edge of the board. This could then also be argued to be a very small Two-Headed Dragon.
The square structure can be recognized in many formations that occur in actual play. Below are some constellations of stones forming partial formed squares, that occur frequently in games.
Preventing the opponent from forming life in certain parts of the board is easy: just occupy the space there yourself. That being said, you could also utilize the fact that missing corner stones in squares forces the necessity of squares to have more connections over to other squares in order to have integrity for forming life. And playing this way is often the case in the game of Go.
A single stone on the side of a square simply prevents it from becoming a life giving square. It may happen that a large form is built around it, but that is for now outside the scope of this section.
If the opponent cannot occupy side positions, then maybe two or more corner stones can be occupied, and thereby forcing that more connections from this squares must be constructed. Each section of a square that is placed between two corner that is occupied by the opponent, must be joined as one, and they need to connect to an external square in order to ensure integrity for this square.
The prevention methods and forcing more connections method can be recognized in various situations
The opponent is preventing two squares from being formed in this situation, by occupying the overlapping side positions of both squares. Since the squares have no possibility of connecting to other remotely positioned squares, they cannot ensure integrity for their own life; hence they are dead. Playing on the position yourself would of course ensure integrity for life with two fully formed single connected squares. We could actually omit some corner stones also, but for clarity I have left that out here.
N.B.: I have added the method for preventing the squares in the lower section of the diagram below. A single white stone is enough to prevent Black completing his two squares.
A well known life and death problem with three overlapping positions in the corner has the same solution as above.
This can also be recognized in a familiar life and death problem at the side of the board. Whoever plays first on the marked position ensures the corner.
The next example has four overlapping positions. A single opposing stone on one of the intersecting side stones (the second is marked ), and your two squares cannot be formed. The preventing method used for the situation is placed to the left and below the construction with the two overlapping squares. The stones marked with are the ones that are omitted by the empty center of the other square take priority over being filled.
This problem also arises in the corner, when the opponent plays on the 1-2 point, and prevents the two squares from being formed.
In the situation with just one overlapping position there is not much to say about how to prevent one of the squares. Either occupy a side position or occupy two corner stones. (But the one in the middle)
But there is something else to notice about this position. Both squares have two connections to the other square. From two side stones (closest to the middle) it is possible to run over to the other square. So it should be possible to omit two corner stones for each square. But only one corner stone between each pair of connections. So one of the stones must be the center stone, and the other can freely be selected among the other three corner stones in each square.
When the center stone is removed it also becomes a point, but if you look at it, it has no corner stones. So it must have four connections (by the definition above). And indeed it has. It has two connections to the upper square and two connections to the lower square, since all four side stones of the new square in the middle have shared side stones with other squares.
Corner stones that also are center positions of squares are obliged to be left out in order to ensure integrity. The next example has the opposite property. Here corner stones that also are side stones may not be omitted.
With two overlapping positions there are two connections between the two squares, since one square’s side stone is one of the other square’s corner stones. But placing side stones takes priority over omitting corner stones. So for each square only one corner stone can be omitted.
If a side stone is omitted anyway then we are moving over to rectangles instead, and that will come later.
For two connected squares some corners can be omitted depending on the number of connections, and if a corner stone is a center position at the same time, then they are forced to be omitted, or if a corner stone is a side stone at the same time, then they are prohibited from being omitted.
Three and four squares can also share one side position, and if the opponent get to play there first then all squares are prevented from being completed.
Playing in a way to force more connections will also kill a group if there is no way to make the necessary connections. The forcing method is shown on the right in the diagram
This way of killing can also be found in a life and death problem on the side of the board. Remember that the (a) square needs two connections after White plays the marked positions, because the edge counts as one corner stone has unwillingly been omitted, and it now require two connections to other squares to become a valid square (by the definition above)
If Black could make the connection to another square then he would survive. (N.B. Now you would probably argue that it would be better for White to play at 1. And, yes, that would be better in this diagram, but I am leaving it as it is for the clarity of the two square theory)
Preventing rectangles requires 2 or more stones. Looking closely, you will see that there are some similarities to how squares are prevented. If you simply cover one or the other 3 stones end of the rectangle shape you may recognize how squares are prevented also.
Black’s four-space territory is killed by White with two stones, as in the diagram below. Both ends of the Black formation form a rectangle, and the White dagger simply kills the Black formation by using the same dagger for both rectangles.
One of the solutions Black would like to play in the L-group is to make a square and a rectangle with the two 1-2 & 2-1 marked positions . So if Black plays one of them then White should prevent Black’s square in the corner by playing the other marked position.
If Black plays 1, then White prevent the square in the corner with 2. I have added the preventive method to the right on the board.
If Black continues like this with 3, then White drives a dagger into the heart of the rectangle with 4. I have added the preventive method of forming the rectangle to the right on the board.
Another solution that Black can aim for is to make the rectangle in corner and the square in the bend of the L.
If Black plays first in the J-group he can quickly create multiple possibilities for two squares with a stone on any one of the circles. So the question is if Black can survive when White plays first.
White bends at , and Black tries to make two squares with his stone at , but White prevent this with .
Black then tries to make two rectangles with @(3,1) and @(2,2), White and prevent him from completing, because Black has to put his stones in self atari.
Another possibility is to create a large rectangle and a square in this way with and , but and will again prevent this.
In this problem, White's stones are encircled by Black's rim of stones. And whether White live or dies depends on who plays first.
White can create an rectangle shape with 1. The second eye can be ensured by playing right or left of the lower single white stone. So that is miai.
However if Black plays first, then he can first play atari by throwing in at 1, and start forming the shown zig-zag preventing method. When White takes the stone with 2, then Black play atari from outside with 3. The square-preventing shape is shown in the second diagram below.
Black to play in every diagram
Most problems use these ways (rotated and possibly reaching beyond the board) to prevent squares and rectangles from being formed.
Make sure that Black can form a square and a rectangle in the corner. So where is the crusial common spot between the two diagrams?
Sekis can be constructed in different ways
- A Black string and a White string of stones that have as their only liberties a shared rectangle
- A Black string and a White string of stones that have as their only liberties 2 shared squares
- A Black string and a White string of stones that have as their only liberties 1 shared square, but also 1 independent square each
- Multiple Black and/or White strings that share 2 squares, but with different opponent strings
- Multiple Black and/or White strings that share 2 squares, but with different opponent strings, that terminate with 2 independent squares.
- Seki with stones in Atari
- (At least 2 more ways, that I will find examples for later)