As for definition 3, well, it is badly worded and doesn't state what you think it does. What are you trying to say? Whatever you meant by it, it certainly was not this:
Not being largest successor set is:
(successor set AND not largest successor set) OR (base set).
If nothing else, the thing you are trying to define (largest successor set) appears in the definition.
"Not being largest successor" set is not the definition of largest successor set, but it is the use of the term largest set as part of the expression (successor set AND not largest successor set) "after" it is defined.
You are mixing between
the use of the term "largest successor set" in some expression ("after" it is defined) and
the definition of the term "largest successor set" which is:
Definition 3: Given a successor set in some set, it is called largest successor set iff given a base set in that set, it has at least one successor set that does not have its successor set, in that set.
What is written after the iff does not use the term "largest successor set" in order to define "largest successor set".
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I agree with you that the expression (successor set AND not largest successor set) is misleading, so let's correct it as follows:
If x is largest successor, then M(x) : false.
If x is not largest successor, then M(x) : true.
Not being largest successor set in N is:
(successor set that has its successor set) OR (base set).
In case of N as a member of N, N is a (successor set that has its successor set), which means that "every set", "all sets", ∀ etc. are not related to set N.