Double domination critical and stable graphs upon vertex removal

In a graph a vertex is said to dominate itself and all its neighbors. A double dominating set of a graph G is a subset of vertices that dominates every vertex of G at least twice. The double domination number of G, denoted $γ_{×2}(G)$, is the minimum cardinality among all double dominating sets of G. We consider the effects of vertex removal on the double domination number of a graph. A graph G is $γ_{×2}$-vertex critical graph ($γ_{×2}$-vertex stable graph, respectively) if the removal of any vertex different from a support vertex decreases (does not change, respectively) $γ_{×2}$(G). In this paper we investigate various properties of these graphs. Moreover, we characterize $γ_{×2}$-vertex critical trees and $γ_{×2}$-vertex stable trees.