# Distributions with equal Renyi Entropies

Suppose we have two distributions given by the vectors $p=(p_1,\dots,p_n)$ and $q=(q_1,\dots,q_n)$, with $p_i,q_i\geq 0$, and $\sum_i p_i = \sum_i q_i=1$.

Now suppose that for some $\alpha\in(0,\infty)$,

$$H_{\alpha}(p)=H_{\alpha}(q),$$

where $H_{\alpha}(p)=\frac{1}{1-\alpha}\log\sum_i p_i^{\alpha}$ is the Rényi entropy of $p$. What can we say about $p$ and $q$? Of course if $q$ is a permutation of $p$, then their entropies will be equal. But is the converse true?

• The set of probability distributions $p$ is an $(n-1)$ dimensional region (a simplex). The set of distributions which satisfy $H_a(p)=C$ for some constant $C$ should be an $(n-2)$ dimensional set, since $H_a(p)=C$ is a single constraint. Therefore, there should be a large number of distributions which have the same Renyi entropy for any particular $\alpha$. However, if $H_\alpha(p)=H_\alpha(q)$ for all $\alpha>0$, you could probably conclude $p$ is a permutations of $q$. – Mike Earnest Jun 6 '18 at 13:51

As pointed out in the comment, all you can say is that the ${\alpha}-$ norm of the two vectors are equal: $$\sum_{i=1}^n p_i^{\alpha}=\sum_{i=1}^n q_i^{\alpha}$$ if and only if $$\lVert p\rVert_{\alpha}=\lVert q\rVert_{\alpha}.$$ Of course their $1-$norm is one (e.g., $\sum_{i=1}^n p_i=1$) and their entries are nonnegative as well. So they all lie in the positive orthant of $\mathbb{R}^n$ on the unit sphere $S_{n-1}.$