Grid-Based Graphs, Linear Realizations and the Buratti-Horak-Rosa Conjecture

Abstract

Label the vertices of the complete graph $K_v$ with the integers $\{0, 1, \ldots, v-1\}$ and define the {\em length} $\ell$ of the edge between distinct vertices labeled $x$ and $y$ by $\ell(x,y) = \min( |y-x|, v - |y-x| )$. A {\em realization} of a multiset $L$ of size $v-1$ is a Hamiltonian path through $K_v$ whose edge labels are $L$. The {\em Buratti-Horak-Rosa (BHR) Conjecture} is that there is a realization for a multiset $L$ if and only if for any divisor $d$ of $v$ the number of multiples of $d$ in $L$ is at most $v-d$. We introduce ``grid-based graphs" as a useful tool for constructing particular types of realizations, called ``linear realizations," especially when the multiset in question has a support of size 3. This lets us prove many new instances of the BHR Conjecture, including those for multisets of the form $\{1^a, x^b, y^c \}$ when $a \geq x+y - \epsilon$, where $\epsilon$ is the number of even elements in $\{ x,y \}$, and those for all multisets of the following forms for sufficiently large $v$ with $\gcd(v,y) = 1$ for all $y \in L$: $\{1^a, 2^b, x^c\}$, except possibly when $a \in \{1,2\}$ and $x$ is odd, $\{1^a, x^b, (x+1)^c\}$. This establishes that there are infinitely many sets $U$ of size 3 for which there are infinitely many values of $v$ where the BHR Conjecture holds for each multiset with support $U$. We also show that the BHR Conjecture holds for $\{1^a,x^b,(x+1)^c\}$ when $x \in \{7,9,10\}$ and $\gcd(v,x) = \gcd(v,x+1) = 1$.