The Simultaneous Fractional Dimension of Graph Families

A subset $S$ of the vertices $V$ of a connected graph $G$ resolves $G$ if no two vertices of $V$ share the same list of distances (shortest-path metric) with respect to the vertices of $S$ listed in a given order. The choice of such an $S$ in $V$ amounts to selecting a binary valued function $g$, said to be a resolving function, on $V$. The notion of a fractional resolving function is obtained by relaxing the codomain of $g$ to be the unit interval. Let $|g|=\sum_{v\in V}g(v)$. Given a finite collection $\mathcal{G}$ of connected graphs on a common vertex set $V$, the simultaneous metric dimension of $\mathcal{G}$ is the minimum cardinality of $|S|$ over all $S$ which resolve each member graph of $\mathcal{G}$. In this paper, we initiate the study of simultaneous fractional dimension ${\rm Sd}_f(\mathcal{G})$ of a graph family $\mathcal{G}$, defined to be the minimum $|g|$ over all functions $g$ each resolving all members of $\mathcal{G}$. We characterize the lower bound and examine the upper bound satisfied by ${\rm Sd}_f(\mathcal{G})$. We examine ${\rm Sd}_f(\mathcal{G})$ for families of vertex transitive graphs and for pairs $\{G,\overline{G}\}$ of complementary graphs, determining ${\rm Sd}_f(G,\overline{G})$ when $G$ is a tree or a unicyclic graph.