example of a concave function with $\lim_{x\to 0^+}\frac{g(x)}{-x\ln x}=\infty$ which fullfills some additional condition - MathOverflow

example of a concave function with $\lim_{x\to 0^+}\frac{g(x)}{-x\ln x}=\infty$ which fullfills some additional condition

2012-12-04T09:13:05Z

I'm looking for the example of a concave function $g\colon [0,1]\mapsto \mathbb{R}$, with $g(0)=0$, for which

$\lim\limits_{x\to 0^+}\frac{g(x)}{-x\ln x}=\infty$ and
$\lim\limits_{x\to 0^+}\frac{\lambda g(x)}{g(\lambda x)}=1$ for every $\lambda>1$

Answer by Alexandre Eremenko for example of a concave function with $\lim_{x\to 0^+}\frac{g(x)}{-x\ln x}=\infty$ which fullfills some additional condition

2012-12-04T13:51:48Z

Take $g_1(x)=x\log^2x$. Properties 1,2 are evidently satisfied, and computation of the second derivative shows that it is negative for $0< x<1/e$. Now rescale: $g(x)=g_1(x/e)$.