Combat Series 6 - Corner Joseki mistakes

 

I was recently reading the rather excellent advice on Joseki learning in 'The Fundamentals of Go' and after reading that had encountered exactly the sort of situation he discusses in the equally excellent 'Mastering Basic Corner Shapes' by Young Sun Yoon (recommended)

It's been a day of discovery even allowing for the fact that I only just noticed the corner book gives 3 stones variations and the last tends to be the board position where the starting point of the other 3 comes from. Ah, clever.

Anyway the particular problem was the following (the 3rd in the series, black to play to kill)

Now I considered and discarded the 1 space jump leading to a Ko

And quickly enough discarded extending on the left

As White lives. Settling eventually on the solution as follows

As par the Fundamentals of Go I then started to look a the reasons for each - 1 is an attempt to connect to E2 so D2 blocks if White plays anywhere else Black plays D2 and is connected and White is dead. The A4 throw in removes eye space and then the extension to A1 makes a killing shape and prevents a minor Ko.

All good so far but I then started playing around with weirder White plays. For example

SDKs and above probably know the answer already but this caused me some head scratching. If Black takes

White Blocks (and captures a single stone) then Black has to connect to get a bent 4 in the corner but it is far too early. White forces it immediately

If Black does not take White is alive so Black takes

But it is to early, White has 2 outside liberties and can take A1 and live.

So mis-playing by Black when White plays off Joseki can cause disaster!

Fortunately after some thought (and it was a lot of thought) I came up with the correct response

Basically Black says I don't care - if White takes the 2 bottom stones Black plays A6 and White has 1 eye and if white takes the 2 left side stones Black throws in again at A4 and White again only has 1 eye.

What also interested me was the points and whether it is ever worth White forcing this looking for the mistake (some players, especially at DDK level) are mistake hunters after all.

So base position left alone after Black plays gets its 3 in a line is 21 points with 1 capture so 20 up

If White goes whole hog on the A2 'try to trip up' then Black is also 20 up so White should try for the trip up (if no valid move elsewhere exists). If that plays out White ends up 7 up so that's a 27 point split. Since there is no point damage to trying for the mistake the only factor is whether there are any other moves elsewhere that are Sente and whether this could be left for some Ko threats. Sometimes it's fun to dig into Joseki..