Wednesday, August 10, 2016

analysis - Prove $displaystylelim_{nto infty}{nb^n}=0$

Prove $\displaystyle\lim_{n\to \infty}{nb^n}=0, 0

One last thing I used a certain theorem in my proof which I will give.





Theorem 3.1.10 Let $(x_n)$ be a sequence of real numbers and let $x\in \mathbb{R}$ If $(a_n)$ is a sequence of positive real numbers with $\lim(a_n) = 0$ and if for some constant $C > 0$ and some $m \in\mathbb{N}$ we have $|x_n-x|\leq Ca_n$ for all $n > m$,
then it follows that $lim(x_n ) = x$.




Here is how my proof goes. Though I don't think its right.



Since $00$ and $n \in
\mathbb{N}$ It follows that $nb^n=\frac{n}{(1+h_n)^n}$. Then by Bernoulli's inequality we get $\frac{n}{(1+h_n)^n}\leq \frac{n}{(1+nh_n)}\leq \frac{n}{nh_n}$ By the theorem above we let $C=\frac{n}{h_n}$ and $a_n=\frac{1}{n}$. Thus $\displaystyle\lim_{n\to \infty}{nb^n}=0$

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