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Let $n \in \mathbb N$ and let $\sigma,\tau \in {\rm Sym}(n)$. I am looking for a permutation $x \in {\rm Sym}(n)$ that minimizes the Hamming distance between $x^2 \sigma$ and $\tau x$. Here, the Hamming distance between permutations $a,b \in {\rm Sym}(n)$ is defined as:

$$d(a,b) = \#\{1 \leq i \leq n \mid a(i) \neq b(i) \}.$$

Conjecture: For all $\varepsilon>0$, there exists $n_0 \in \mathbb N$, such that for all $n \geq n_0$ and $\sigma,\tau \in {\rm Sym}(n)$, there exists $x \in {\rm Sym}(n)$ with $$d(x^2 \sigma,\tau x)\leq \varepsilon n.$$

I have tried to find solutions to this problem for $n=50$ and random $\sigma,\tau$ with various approaches. The outcome was that $x \in {\rm Sym}(n)$ with $d(x^2 \sigma,\tau x)\leq 8$ could be found in almost all cases. I found this encouraging but of course it does not mean much with regard to the conjecture - but maybe it indicates that this is an interesting problem.

Of course there are many possible generalizations of this question.

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  • $\begingroup$ In the special case where $\tau$ and $\sigma$ commute, the choice $x=\tau\sigma^{-1}$ gives distance zero, So maybe the hard cases are the ones where $\tau$ and $\sigma$ are quite far from commuting. $\endgroup$ Commented Jul 16 at 8:34
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    $\begingroup$ On the "many possible generalizations": given $w \in F_2$ one may ask whether the map $\operatorname{Sym}(n) \to \operatorname{Sym}(n), (x,\sigma) \mapsto w(x,\sigma)$ has uniformly cobounded image for all $n$ and all $\sigma \in \operatorname{Sym}(n)$. The answer is obviously yes for $w = x\sigma$ and obviously no for $w = x\sigma x^{-1}$, and the question is the case $w = x^2 \sigma x^{-1}$. $\endgroup$ Commented Aug 29 at 12:34

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