Two-Headed Dragon

    Keywords: Life & Death

Chinese 1: 假眼活 (jia3 yen3 huo2, literally "false eye life")
Chinese 2: 兩頭蛇 (liang3 tou2 she2, literally "two-headed snake")
Japanese: 欠け眼生き (kakeme iki)

[Diagram]

Example 1

A Two-Headed Dragon (also known as Dragon Eats its Own Tail) is an unusual method of creating a living group. A Two-Headed Dragon is a group that has two eyes that are at first sight false eyes, nevertheless these two eyes are enough for living. To get this position, the group has to be a ring around some living group of an opponent, as in this diagram. Because the eyes are connected to each other in both directions, White cannot play at either of them first, and thus they function as real eyes, even though they have all the looks of false ones [1]. In actual play, these positions are extremely rare but not non-existent.

You can download a [ext] game containing a two-headed dragon.

Two other examples, one a game from the 20th World Amatuer Championship and one a go problem, are [ext] located on the GoGoD site.


More examples

[Diagram]

Example 2

[Diagram]

Example 3

Note: Black is the one with the Two-Headed dragon (or maybe a "Four-Headed dragon"!)

[Diagram]

Example 4


Comments

The "seemingly false eyes" in the two-headed dragon patterns are not false eyes, of course, but real eyes. "False eyes" are arrangements of stones which can look like eyes to the freshly initiated, but aren't. There is a simple test that unmasks the majority of the false eyes (see recognizing an eye and false eye). However, in the above cases the simple test fails and the eyes are indeed real.

Hence, the concept of a two-headed dragon really is subjective. To someone with a formal understanding of eye, it is just another example of a live group.

An example of a similar pattern which does not surround a living opponent group would be:

[Diagram]

X Similar idea

If the eyes in a two-headed dragon are to "seem false" then these should too.

fractic: Those look like real eyes.

[Diagram]

X Similar idea

fractic: Look at the diagram on the left. The top-right eye is a real eye because black controls all four orthogonally adjacent intersections (marked with squares), and 3 of the 4 diagonally adjacent intersections (marked with circles). Black controls circle because white can never play a living stone there.

[Diagram]

detail of a two-headed dragon's eye

fractic: The diagram on the left is a part of the first example. The diagonal intersections around the eye are again marked with circles. As you can see Black only controls two of them, White controls the other two. That's why the eyes of a two-headed dragon look false at first glance.

Beginner: Why is this a two-headed dragon? Can't white capture everything by just playing in the eye?

fractic: The diagram on the left is only a part of a two-headed dragon. It's the lower right eye of the first example.


See also


This is a copy of the living page "Two-Headed Dragon" at Sensei's Library.
(OC) 2012 the Authors, published under the OpenContent License V1.0.
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