- #1
C0nfused
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Hi everybody,
I have a question concerning limits of 2-variable real functions and complex functions of 1 complex variable. When we have a function f:A->R, (A<=RxR and (0,0)εΑ) and we are looking if the limit of f at (0,0) exists, then we can do a substitution to polar co-ordinates like this: (X,Y)->(Rcost, Rsint) and look for the limit of f(Rcost,Rsint) when R->0. If the limit doesn't exist or exists, but is a function of t, then we say that the limit of f(X,Y) when (X,Y)->0 doesn't exist. If the f(Rcost, Rsint), R->0 exists and is a constant, then it may be the limit of f(X,Y), but we have to check it by definition.
I think all the above are correct, so my question is, why do we have to check by definition the limit in the last case? I have found examples that the limit doesn't exist, although we can find a constant one with polar co-ordinates, but I want a more precise explanation than examples.
I have read that, by using this change of variables, we actually are looking for the limit in the straight lines Y(X)=Xtant , which seems quite sensible, but on the other hand, a mathematician who probably has misunderstood this specific topic, told me that if we find a constant limit, then it is correct, beacuse every point of the plain can be represented by polar co-ordinates, so depending on t, we can reach (0,0) through any line. And the "problem" was that I was not sure what to tell him to prove him wrong...
(All the above apply to 1-complex-variable functions, too)
To end up, I don't think i have a good grasp of the subject so any explanation would help. Sorry for the size of the post.
Thanks
I have a question concerning limits of 2-variable real functions and complex functions of 1 complex variable. When we have a function f:A->R, (A<=RxR and (0,0)εΑ) and we are looking if the limit of f at (0,0) exists, then we can do a substitution to polar co-ordinates like this: (X,Y)->(Rcost, Rsint) and look for the limit of f(Rcost,Rsint) when R->0. If the limit doesn't exist or exists, but is a function of t, then we say that the limit of f(X,Y) when (X,Y)->0 doesn't exist. If the f(Rcost, Rsint), R->0 exists and is a constant, then it may be the limit of f(X,Y), but we have to check it by definition.
I think all the above are correct, so my question is, why do we have to check by definition the limit in the last case? I have found examples that the limit doesn't exist, although we can find a constant one with polar co-ordinates, but I want a more precise explanation than examples.
I have read that, by using this change of variables, we actually are looking for the limit in the straight lines Y(X)=Xtant , which seems quite sensible, but on the other hand, a mathematician who probably has misunderstood this specific topic, told me that if we find a constant limit, then it is correct, beacuse every point of the plain can be represented by polar co-ordinates, so depending on t, we can reach (0,0) through any line. And the "problem" was that I was not sure what to tell him to prove him wrong...
(All the above apply to 1-complex-variable functions, too)
To end up, I don't think i have a good grasp of the subject so any explanation would help. Sorry for the size of the post.
Thanks