During my research I stumbled upon this system over $\mathbb{Z}_{2^n}$. Let $k=2^{n-1}-1$ then the system looks like
\begin{equation*}
\begin{cases}
x_1+2^{n-1}=d_k +x_k \\
x_2+2^{n-1}=d_1+x_1 \\
x_3+2^{n-1}=d_2+x_2 \\ \tag{1}
\dots \\
x_k+2^{n-1}=d_{k-1}+x_{k-1} \\
x_i \notin \{x_j,x_j + 2^{n-1}\}, i \neq j \\
d_i \notin \{d_j,-d_j\}, i \neq j,
\end{cases}
\end{equation*}
where '+' means addition in $\mathbb{Z}_{2^n}$ and $x_i,d_i$ are variables for i $\in \overline{1,k}$.
I've noticed, that system (1) without any conditions has solutions iff $d_1+d_2+\dots+d_k=2^{n-1}$, so my idea was to fix $d_1,d_2,\dots d_k$ and then the solution to the system without coniditons looks like
\begin{equation}
\begin{cases}
x_1=d_k +2^{n-1} \\
x_2=d_k+d_1 \\
x_3=d_k+d_1+d_2 +2^{n-1} \\ \tag{2}
\dots \\
x_{k-1}=d_k+d_1+\dots +d_{k-2} \\
x_k=2^{n-1}.
\end{cases}
\end{equation}
Now, let $c_i=d_i+2^{n-1}$, then if $d_i \notin \{d_j,-d_j\}, i \neq j$ it follows that $c_i \notin \{c_j,-c_j\}, i \neq j$. Then in terms of $c_i$ solution (2) look like
\begin{equation}
\begin{cases}
x_1=c_k \\
x_2=c_k+c_1 \\
x_3=c_k+c_1+c_2 \\ \tag{3}
\dots \\
x_{k-1}=c_k+c_1+\dots +c_{k-2} \\
x_k=0.
\end{cases}.
\end{equation}
So, solving of system (1) is equivalent to finding $c_1,\dots ,c_k$, such that $c_i \notin \{c_j,-c_j\}, i \neq j$ and all sums like $c_i+\dots +c_j, 1 \leq i \leq j \leq k-2$ are nonzero modulo $2^{n-1}$.
I've programmed it and was able to find some interesting candidates for $C=\{c_i\}$. It is $C=\{1,2,\dots,2^{n-2}-1,2^{n-1},2^{n-1}+1,\dots,2^{n-1}+2^{n-2}\}$ as for $n=4$ we have solutions like
$(c_1,c_2,\dots,c_7)=(3, 2, 1, 9, 10, 11, 12)$
$(c_1,c_2,\dots,c_7)=(1, 2, 3, 12, 11, 10, 9)$ and a couple of others.
For $n=5$ we have something like
$(c_1,c_2,\dots,c_{15})=(24, 19, 20, 18, 21, 22, 17, 23, 1, 2, 3, 4, 5, 6, 7)$
$(c_1,c_2,\dots,c_{15})=(1, 6, 23, 4, 19, 7, 24, 21, 22, 20, 3, 18, 2, 17, 5)$.
For $n=6$ my program unfortunately wasn't able to find any. So the question is, how to find solutions to (1) for any $n \in \mathbb{N}$ or at least prove its existence? Maybe there are other ways much simplier ways to solve (1) rather than mine? I'll appreciate any help.
2026-05-05 12:26:53.1777984013