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Let $f\in C^1[0,a]$ and $f(0)=0$. Is it true that

$\int_0^a \left(\sqrt{x}f(x)\right)^{\prime} \left(\frac{f(x)}{\sqrt{x}}\right)^{\prime}\, dx\geq 0\;\;?$

What is the general form of this type inequality?

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    thanks Fabian, yes your example works.. and your suggestion too :)2011-06-03

2 Answers 2

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In the form \int_0^a \big(({f(x) \over 2x})^2 - (f'(x))^2\big) \, dx \leq 0 this follows readily from Hardy's Inequality for $p = 2$. See http://en.wikipedia.org/wiki/Hardy%27s_inequality for this and generalizations.

To apply it, let your |f'(x)| be what is called $f(x)$ in that article. Also you need to extend f'(x) to be zero for $x \geq a$ to apply the theorem.

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I have found a particular proof for this (which is stronger since i proved $\int_0^a f'(x)^2\, dx \geq \int_0^a \left(\frac{f(x)}{2x}\right)^2 \, dx$). since $f(0)=0$ we have \lim_{x\rightarrow 0} \frac{f(x)^2}{x}=\lim_{x\rightarrow 0} 2f'(x)f(x)=2f'(0)f(0)=0.

so integration by part gives us \int_0^a \left(\frac{f(x)}{2x}\right)^2 \, dx + \frac{f(a)^2}{4a}= \int_0^a \frac{f'(x)f(x)}{2x}. By AM-GM we have f'(x)^2 + \frac{f(x)^2}{4x^2} \geq \frac{f(x)f'(x)}{x}

integrate this inequality from $[0,a]$ and using 2 \int_0^a \left(\frac{f(x)}{2x}\right)^2 \, dx + \frac{f(a)^2}{2a} = \int_0^a \frac{f'(x)f(x)}{x}\, dx we are done.