Wednesday, 5 November 2014

calculus - Equivalent theorems for substitution of variables in limits

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 limxcf(x)=l exists whith c,lR{+,}, than let g be a function defined in a neighbourhood I(l) besides (at most) the point l, such that:




.If lR, g is continous in l



.If l={+,}, the limit limylg(y) exists



Then limxcg(f(x))=limylg(y)



The second one:



Suppose that limxcf(x)=l and limylg(y)=m exist whith c,l,mR{+,} and that, if lR, there exists a neighbourhood I(c) in which f(x)l xI(c) and xc




Then again limxcg(f(x))=limylg(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 xI(c) and xc it is related to existance of the limit.



Can anyone help me with this?



Thanks a lot for your help

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