I have some doubts on the theorem for substitutions of the variable in limits. There are two equivalent theorems for this.
The first one:
Suppose that limx→cf(x)=l exists whith c,l∈R∪{+∞,−∞}, than let g be a function defined in a neighbourhood I(l) besides (at most) the point l, such that:
.If l∈R, g is continous in l
.If l={+∞,−∞}, the limit limy→lg(y) exists
Then limx→cg(f(x))=limy→lg(y)
The second one:
Suppose that limx→cf(x)=l and limy→lg(y)=m exist whith c,l,m∈R∪{+∞,−∞} and that, if l∈R, there exists a neighbourhood I(c) in which f(x)≠l ∀x∈I(c) and x≠c
Then again limx→cg(f(x))=limy→lg(y)
I don't understand in particular how the second theorem can be equivalent to the first one, and it is not clear to me how the condition for which exists a neighbourhood I(c) in which f(x)≠l ∀x∈I(c) and x≠c it is related to existance of the limit.
Can anyone help me with this?
Thanks a lot for your help
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