Consider any graph with $n$ vertices, arbitrary size and no loops. Take a random labeling $i=1,...,n$ of the vertices. Is there a way of relabeling the graph such that, for any vertex $i$, with corresponding new label $\ell_i\in\mathbb{Z}$, there exists a set $L$ for which $$ \deg(i)=\sum_{j=1}^n \textbf{1}_L(\ell_i-\ell_j) $$ yields the degree of vertex $i$? Here, $\textbf{1}_L$ is the indicator function on the set $L$.

For example, in the cycle case, with $\ell_i=i$, the set $L=\{\ell\,:\,|\ell|\equiv 1 \mod (n-2)\}$ seems to do the trick, that is, $\deg(i)=2, \forall i$. Any ideas regarding a general graph?

  • $\begingroup$ Must the labels $\ell_1, \dots, \ell_n$ be in the set $1, 2, \dots, n$? $\endgroup$ – Misha Lavrov Jan 25 at 21:05
  • $\begingroup$ No, I'm considering $\ell_i\in \mathbb{Z}$. $\endgroup$ – sam wolfe Jan 25 at 21:17

The most straightforward thing to do is take the labels $\ell_1, \dots, \ell_n$ to be so far apart that no difference $\ell_i - \ell_j$ is repeated.

This is achieved by any Sidon sequence, and the densest ones with $n$ elements would have labels going up to about $n^2$ or so. If we don't care for optimizing, then we can take $\ell_i = 2^i$; if $2^a - 2^b = 2^c - 2^d$, then we also have $2^a + 2^d = 2^b + 2^c$, and by the uniqueness of binary representations, we must have $a=c$ and $b=d$.

Now just let $L = \{2^i - 2^j : ij \in E\}$, where $E$ is the edge set of the graph. This leads to $$ \sum_{j=1}^n \mathbf1_L(\ell_i -\ell_j) = \sum_{j=1}^n \mathbf1_E(ij) = \deg(i). $$

  • $\begingroup$ What is the set $E$? $\endgroup$ – sam wolfe Jan 25 at 22:57
  • $\begingroup$ The edge set of the graph. $\endgroup$ – Misha Lavrov Jan 25 at 22:57
  • $\begingroup$ Thank you, it's very clear now! Interesting concept this of Sidon sequence. $\endgroup$ – sam wolfe Jan 25 at 23:15

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