Not every graph can be reconstructed from its boundary distance matrix

Document Type : Original paper

Authors

1 CDTIME and Departamento de Matemáticas, Universidad de Almería, Almería, Spain

2 Departament de Matemàtiques, Universitat Politècnica de Catalunya, Barcelona, Spain

Abstract

A vertex $v$ of a connected graph $G$ is said to be a boundary vertex of $G$ if for some other vertex $u$ of $G$, no neighbor of $v$ is further away from $u$  than $v$. The boundary  $\partial(G)$ of  $G$ is the set of all of its boundary vertices. The boundary distance matrix $\hat{D}_G$ of a graph $G=([n],E)$ is the square matrix of order $\kappa$, being $\kappa$ the order of $\partial(G)$, such that for every $i,j\in \partial(G)$,  $[\hat{D}_G]_{ij}=d_G(i,j)$. In a recent paper [doi.org/10.7151/dmgt.2567], it was shown that if a graph $G$ is either a block graph or a unicyclic graph, then $G$ is uniquely determined by the boundary distance matrix $\hat{D}_{G}$  of $G$, and it was also conjectured that this statement holds for every connected graph $G$, whenever both the order $n$ and the $|\partial(G)|$ (and thus also the boundary distance matrix) of $G$ are prefixed (i.e., the set of boundary vertices is known as a subset of $V$, but not their identities). After proving  that this conjecture is  true for several graph families, such as being of diameter 2, having  order at most $n=6$ or being Ptolemaic, we show that this statement does not hold when considering, for example, either  the family of split graphs of diameter 3 and order at least $n=10$ or the family of distance-hereditary graphs of order at least $n=8$.   

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