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How to prove this inequality: $\dfrac{n(\ln(n)-2\ln(2))}{2\ln(n)(\ln(n)-\ln(2))^2} > 1$ for all $n\ge20$

I tried to apply this approach but I get a large first differentiate $u'(x)$ whose sign is not easy to determine

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    Arjang, I also suggested induction on pedja's earlier question http://math.stackexchange.com/questions/63283/proof-that-lnn2-lnn-1-n-for-all-n-in-mathbbn, but when mixedmath interrogated me on chat it became apparent that some calculus is required anyway. I wonder what is an elementary way to solve these types of problems with a particular base case (i.e. $n \ge 20$)?2011-09-14

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Idea: The goal is to show that $f(x)$ is increasing. We could take the derivative, but in this case that will be tedious. Instead use logarithms. We have that $f(x)$ is positive and increasing if and only if $\log f(x)$ is increasing, so all we need to do is look at the logarithmic derivative and the value at $n=20$.

Proving the Inequality: Rewriting your above fraction, let $g(n)=\frac{n\log\left(\frac{n}{4}\right)}{\log\left(n\right)\left(\log\left(\frac{n}{2}\right)\right)^{2}}.$ then $\log\left(g(n)\right)=\log n+\log\log\left(\frac{n}{4}\right)-\log2-\log\log n-2\log\log\left(\frac{n}{2}\right).$ Taking the derivative we see that \frac{g^{'}(x)}{g(x)}=\frac{1}{x}+\frac{1}{x\log\left(\frac{x}{4}\right)}+\frac{-1}{x\log x}+\frac{-2}{x\log\left(\frac{x}{2}\right)}. Now, notice that $\frac{1}{\log\left(\frac{x}{4}\right)}+\frac{-1}{\log x}>0$. Next, for $x\geq20$, $\log\left(\frac{x}{2}\right)\geq\log\left(10\right)>\log(e^{2})=2$ so that $1+\frac{-2}{\log\left(\frac{x}{2}\right)}>0.$ This implies that for $x\geq20$ \frac{g^{'}(x)}{g(x)}>0. As $g(20)>1$ , the desired inequality follows.

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    ,That's sounds better...could you please correct definition of $g(n)$ and I will accept your answer2011-09-14