Tuesday, 12 September 2017

real analysis - For sufficiently large n, Which number is bigger, 2n or n1000?




How do I determine which number is bigger as n gets sufficiently large, 2n or n1000?



It seems to me it is a limit problem so I tried to tackle it that way.



lim



My thoughts are that, after some n, the numerator terms will be more than the terms in the denominator so we'll have something like \frac{\overbrace{2\times 2\times\cdots \times 2}^{1000\text{ factors}}}{n\times n \times \cdots \times n} \times 2^{n-1000}

At this point, I was thinking of using the fact that 2^n grows faster slower than n! as n gets larger so the limit, in this case, will be greater than 1, meaning 2^n is bigger than n^{1000} for sufficiently large n. This conclusion is really just a surmise based on a non-concrete formulation. Therefore, I'd appreciate any input on how to tackle this problem.


Answer



Note that\frac{2^{n+1}}{2^n}=2\text{ whereas }\frac{(n+1)^{1\,000}}{n^{1\,000}}=\left(1+\frac1n\right)^{1\,000}.Since\lim_{n\to\infty}\left(1+\frac1n\right)^{1\,000}=1<2,you have that\left(1+\frac1n\right)^{1\,000}<\frac32if n is large enough.It's not hard to deduce from this that 2^n>n^{1\,000} if n is large enough.


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