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I am a beginner, so please forgive me if I am making any mistakes. I find it difficult without further explanations. If black is putting 5 and then capturing white 4, in the following turn because of the rule of ko, white cannot place on 4 again. She cannot place on 'a' as well as that will be a suicide. Unless a very important menace elsewhere, how can white survive here?
And again sorry if I did not see something important.
White can survive because of the ko you mention. So white indeed needs an "important menace" elsewhere (a ko-threat) to survive. If there is no such thing, white will die.
However, given the choice between a sequence where white dies unconditionally and one where white only dies if there is no menace elsewhere (conditionally), the first one is clearly the better choice for black, which is why the diagram resulting in ko is labeled a failure.
No, it isn't, it captures White 4.
Beginners often struggle with this idea: that making a ko is different from dying unconditionally. I'd like to make a link from this failure diagram to a page which explains this in detail--but I've not found anything suitable. Surely such a page exists on this site? Bear in mind that people looking at "beginner exercise 2" will mostly be real beginners. The discussion at ko doesn't contain enough detail; Basics on kos? is too abstract, and Basic Kofight Example? is close to what I'm looking for, but the level of discussion on that page would be intimidating for many beginners. So, have I missed something? or do we need to make a new page for this?
I think you are right. We need a Ko - Introductory page. :)
I agree with xela and Bill. A trouble with some of the introductory material on SL is to make the material understandable to people with diverse backgrounds.
It can be very difficult to come out with examples that introduce a topic in a coherently way, without attracting others who would comment that certain moves or aspects are suboptimal, and to present it to a way non-mathematicians can understand. For this reason I tend to write such material more verbosely, but such a writing style sometimes looks like verbal diarrhoea to some of the people with mathematical background. Some of these people like to change the text to something very concise and short, but hard to understand for those who gets put off with symbols and equations.
I'm close to a beginner, and I found this exercise much more difficult than the following ones. Maybe it could be move later in the list of exercises.