Mathematical definition of an eye

[Groin]

The first thing is to agree about what is an eye and that seemed to be unclear in a recent post (sorry that was not long ago but OGF search is not always easy to use)

Ok I got it

Cho Chikun Book - Why is this a false eye?

Hi, why in diagram 46 the upper space ‘a’ is categorized as a false eye? I have in mind that if I surround all the stones of the opponent group, I can put my stone in this space and capture everything. This definition satisfies all diagrams except 46. The only thing I could think of is a domino effect (since the lower ‘a’ is a false eye, immediately the upper one becomes one too, as soon as the 3 lower stones are captured). But if this is the reasoning, in all diagrams the second space should al…