Refusing quadrupal ko

This is a key game from yesterday's Samsung Cup preliminary tournament between Na Hyun 3-dan and Shi Yue 9-dan.

Position at B263 (double-ko at upper-left corner)

At this moment, the position was extremely close. The upper-left corner has a double ko and black is unconditionally alive. Let's have a look how Shi Yue exploited this situation.

aji in the bottom-right corner

White 2 White 4 and White 6 are all forcing moves. Then White 8 is a brilliant tesuji to build a ko shape in the corner.

Possibly a second double ko

After Black 13 white can throw in at 'a' to start another ko. Without the first double ko, this would be hopeless for white. But now white has unlimited ko threats so his group cannot be killed. This situation would be equivalent to a quadrupal ko to void this game. However, Shi Yue had a better plan ......

approach ko

Omitting some exchanges at the upper-left corner (Shi Yue used the double ko to gain extra thinking time in byoyomi), white played forcing move White 1 and White 3 before playing White 5 to form an approach ko (black has no time to capture from the side of 'a'). White needs two extra moves - 'b' and White square - to make it a direct ko. This is normally hopeless. But because of the upper-left-corner double ko, black had to waste a move at Black 6 to eliminate the ko threats there. So this is in fact a one-step approach ko. And white happened to have just enough ko threat to win it.

final result

Ignoring Black 1 , Shi Yue played White 2 to create a direct ko. And White 4 happened to be a suitable ko threat. So Na Hyun had to resign.


Metallian's picture

This is an exampla of the subtlety and finest points of this ancient game. Thanks for explaining it in such an understandable way mace.