Some new general lower bounds for mixed metric dimension of graphs

Let $G=(V,E)$ be a connected simple graph. The distance $d(u,v)$ between vertices $u$ and $v$ from $V$ is the number of edges in the shortest $u-v$ path. If $e=uv \in E$ is an edge in $G$ than distance $d(w,e)$ where $w$ is some vertex in $G$ is defined as $d(w,e)=\min(d(w,u),d(w,v))$. Now we can say that vertex $w \in V$ resolves two elements $x,y \in V \cup E$ if $d(w,x) \neq d(w,y)$. The mixed resolving set is a set of vertices $S$, $S\subseteq V$ if and only if any two elements of $E \cup V$ are resolved by some element of $S$. A minimal resolving set related to inclusion is called mixed resolving basis, and its cardinality is called the mixed metric dimension of a graph $G$. This graph invariant is recently introduced and it is of interest to find its general properties and determine its values for various classes of graphs. Since the problem of finding mixed metric dimension is a minimization problem, of interest is also to find lower bounds of good quality. This paper will introduce three new general lower bounds. The exact values of mixed metric dimension for torus graph is determined using one of these lower bounds. Finally, the comparison between new lower bounds and those known in the literature will be presented on two groups of instances: - all 21 conected graphs of order 5; - selected 12 well-known graphs with order from 10 up to 36.