Sunday, October 21, 2018

limits - "Proving" that 00=1




I know that 00 is one of the seven common indeterminate forms of limits, and I found on wikipedia two very simple examples in which one limit equates to 1, and the other to 0. I also saw here: Prove that 00=1 using binomial theorem
that you can define 00 as 1 if you'd like.



Even so, I was curious, so I did some work and seemingly demonstrated that 00 always equals 1.



My Work:




y=lim



\ln{y} = \lim_{x\rightarrow0^+}{(x\ln{x})}



\ln{y}= \lim_{x\rightarrow0^+}{\frac{\ln{x}}{x^{-1}}} = -\frac{∞}{∞}



\implies Use L'Hôpital's Rule



\ln{y}=\lim_{x\rightarrow0^+}\frac{x^{-1}}{-x^{-2}}
\ln{y}=\lim_{x\rightarrow0^+} -x = 0

y = e^{0} = 1



What is wrong with this work? Does it have something to do with using x^x rather than f(x)^{g(x)}? Or does it have something to do with using operations inside limits? If not, why is 0^0 considered indeterminate at all?


Answer



Someone said that 0^0=1 is correct, and got a flood of downvotes and a comment saying it was simply wrong. I think that someone, me for example, should point out that while saying 0^0=1 is correct is an exaggeration, calling that "simply wrong" isn't quite right either. There are many contexts in which 0^0=1 is the standard convention.



Two examples. First, power series. If we say f(t)=\sum_{n=0}^\infty a_nt^n that's supposed to entail that f(0)=a_0. But f(0)=a_0 depends on the convention that 0^0=1.



Second, elementary set theory: Say |A| is the cardinality of A. The cardinality of the set off all functions from A to B should be |B|^{|A|}. Now what if A=B=\emptyset? There as well we want to say 0^0=1; otherwise we could just say the cardinality of the set of all maps was |B|^{|A|} unless A and B are both empty.




(Yes, there is exactly one function f:\emptyset\to\emptyset...)



Edit: Seems to be a popular answer, but I just realized that it really doesn't address what the OP said. For the record, of course the OP is nonetheless wrong in claiming to have proved that 0^0=1. It's often left undefined, and in any case one does not prove definitions...


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