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Suppose that $f$ is a continuous function on $[0,1]$. For $0<a<1$, if $$ \varlimsup_{\delta \rightarrow 0} \frac{\sup_{0<\lvert y\rvert\leq \delta}\lvert f(x+y)-f(x)\rvert}{\delta^{a}} = \infty, $$ then, given any $\epsilon>0$, is it true that $$ \varliminf_{\delta \rightarrow 0} \frac{\sup_{0<\lvert y\rvert\leq \delta}\lvert f(x+y)-f(x)\rvert}{\delta^{a+\epsilon}} = \infty? $$

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  • $\begingroup$ What about $f(r)=r^{a-\epsilon}$ for some small $\epsilon>0$? $\endgroup$
    – Echo
    Apr 8 at 7:56
  • $\begingroup$ @Echo Its lower limit is infinite. $\endgroup$
    – Watheophy
    Apr 8 at 8:00
  • $\begingroup$ Ok, lets rename it, say $f(r)=r^{a-\delta}$ for some small $\delta>0$? As soon as $\epsilon>\delta$, you're in trouble. Finally, you let $\delta$ shrink with $r$. $\endgroup$
    – Echo
    Apr 8 at 8:02
  • $\begingroup$ @Echo Do you mean $f(r) = r^{a-r}$? $\endgroup$
    – Watheophy
    Apr 8 at 8:07
  • $\begingroup$ @Echo The dominator in the lower limit is $\delta^{a+\epsilon}$. $\endgroup$
    – Watheophy
    Apr 8 at 8:09

1 Answer 1

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This example is not continuous, but one can replace the jumps with linear pieces of fast growing slopes. Fix $0<\epsilon<a$. Define a sequence $(r_j)_{j\in\mathbb N}$ tending to zero inductively as follows: $r_1=1$ and $0<r_{j+1}<r_j$ so small that $$ \frac{r_{j+1}^{a-\epsilon}}{r_j^{a+\epsilon}}\le 1. $$ Then set $$ f(t)=\begin{cases}0&t=0,\\ r_{j+1}^{a-\epsilon}& r_{j+1}<t\le r_j,\\ 1&t>1. \end{cases} $$ We have $$ \limsup\frac{f(t)}{t^a}=\lim_j\frac{f^+(r_{j+1})}{r_{j+1}^a}=\lim_j\frac{r_{j+1}^{a-\epsilon}}{r_{j+1}^a}=\infty $$ while on the other hand $$ \liminf \frac{f(t)}{t^{a+\epsilon}}=\lim_j\frac{f(r_j)}{r_j^{a+\epsilon}}=\frac{r_{j+1}^{a-\epsilon}}{r_{j}^{a+\epsilon}}\le 1. $$

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  • $\begingroup$ Your sequence $r_j$ is kind of weird. Its ratio $r_{j+1}/r_j$ converges faster than $r_j$. Could you please example one sequence like this? $\endgroup$
    – Watheophy
    Apr 8 at 12:11
  • $\begingroup$ Say $r_{j+1}=r_j^{\frac{a-\epsilon}{a+\epsilon}}$. $\endgroup$
    – Echo
    Apr 8 at 12:20

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