2
$\begingroup$

Maple tells me that $\lim_{n \to \infty} n^2 \log^n(1 - \frac{c \log n}{n}) = 0$ for any constant $c$, but I can't find a way to prove it. Any suggestions?

  • 0
    Thanks guys, I clarified the question.2010-10-20

3 Answers 3

6

Well $\log\left(1-c\frac{\log n}{n}\right)=-c\frac{\log n}{n} +O\left(\frac{(\log n)^2}{n^2}\right)$ and so $n^2\log\left(1-c\frac{\log n}{n}\right)^n=(-c)^n\frac{(\log n)^n}{n^{n-2}} \left(1+O\left(\frac{\log n}{n}\right)\right)^n.$ Now $n\log\left(1+O\left(\frac{\log n}{n}\right)\right)=O(\log n)$ so that $\left(1+O\left(\frac{\log n}{n}\right)\right)^n$ grows as at most a polynomial in $n$. The $n^n$ in the denominator will swamp everything else...

4

The Taylor series expansion $- \log (1-x) = \sum_{k \ge 1} \frac{x^k}{k}$ shows that $\log(1 - x) = O(x)$ for small $x$. So $\log \left( 1 - \frac{c \log n}{n} \right) = O \left( \frac{\log n}{n} \right)$, hence the absolute value of the expression in question is bounded by a constant times $\frac{(\log n)^n}{n^{n-2}}$ which rather clearly goes to zero.

This is a good example of why it's more flexible to use big-O notation than to deal directly with the definition of a limit.

  • 1
    This is also *a good example of why* it is dangerous *to use the big-O notation* without the utmost care for the constants hidden therein. Here, *the absolute value of the expression in question* is not a priori bounded by a constant times $\frac{(\log n)^n}{n^{n-2}}$ but only by $c^n\frac{(\log n)^n}{n^{n-2}}$ for some constant $c$. (Fortunately all this is washed away by the denominator $n^{n-2}$.)2011-11-10
1

The case $c=0$ is clear, and then for the rest of its values it's enough to consider and calculate $\lim_{n\rightarrow\infty} n^2 \log^n(1 + \frac{|c| \log n}{n})$. Then we have that: $\lim_{n\rightarrow\infty} n^2 \log^n(1 + \frac{|c| \log n}{n})=\lim_{n\rightarrow\infty} n^2 \left(\frac{|c|\log n}{n}\right)^{n} \left(\left(\frac{\log(1 + \frac{|c| \log n}{n})}{(\frac{|c| \log n}{n})}\right)^{\frac{n}{|c|\log n}}\right)^{|c|\log n}= \lim_{n\rightarrow\infty} n^{2} \left(\frac{|c|\log n}{n}\right)^{n} \frac{1}{e^{\frac{|c|\log n}{2}}}=0.$

The auxiliary limit i resorted to is $\lim_{x\to0} \left({\frac{\ln(1+x)} {x}}\right)^\frac{1}{x}= \frac{1}{\sqrt{e}}.$

The proof is complete.