# The ratio of two $L^p$ norms

Let $f$ be a non-negative function on a measure space $(X,\mu)$ with $\mu(X) = 1$. Is there a known characterization of when

$$\lim_{n \rightarrow \infty } \frac{\|f^n\|_p}{\|f^n\|_q } = 1,$$

for $1 \leq p < q \leq \infty$? Since $\mu$ is a probability measure, we know that this limit is always less than 1, but I am seeking to characterize (in terms of $p$ and $q$) the functions such that the limit is in fact precisely 1.

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$\|f\|_\infty$ finite? –  Did May 6 '12 at 21:38

First of all, for the question to make sense we must have $f\in L^r(X,\mu)$ for $p\le r<\infty$. If, as Didier suggests in his comment, $f\in L^\infty(X,\mu)$, then $$\frac{\|f^n\|_p}{\|f^n\|_q}=\left(\frac{\|f\|_{np}}{\|f\|_{nq}}\right)^n.$$ Since $\lim_{r\to\infty}\|f\|_r=\|f\|_\infty$, $$\lim_{n\to\infty}\frac{\|f\|_{np}}{\|f\|_{nq}}=1.$$ But this does not determine the desired limit.
Consider $f(x)=\chi_A(x)$, the characteristic function of a measurable subset $A\subset X$ with $\mu(A)>0$. Then $$\frac{\|f^n\|_p}{\|f^n\|_q}=(\mu(A))^{1/p-1/q},$$ which can be any number in $(0,1]$. As another example, if $X=[0,1]$ with Lebesgue measure and $f(x)=x^\alpha$, $\alpha>0$, then $$\frac{\|f^n\|_p}{\|f^n\|_q}=\frac{(n\,\alpha\,q+1)^{1/q}}{(n\,\alpha\,p+1)^{1/p}}\to0.$$ My conjecture that for bounded $f$, the limit is one if and only if $f$ is constant almost everywhere.