Predominating a vertex in the connected domination game

Abstract

The connected domination game is played just as the domination game, with an additional requirement that at each stage of the game the vertices played induce a connected subgraph. The number of moves in a D-game (an S-game, resp.) on a graph $G$ when both players play optimally is denoted by ${\gamma }_{\mathrm{cg}}(G)$ (${\gamma }_{\mathrm{cg}}^{\prime }(G)$, resp.). Connected Game Continuation Principle is established as a substitute for the classical Continuation Principle which does not hold for the connected domination game. Let $G|x$ denote the graph $G$ together with a declaration that the vertex $x$ is already dominated. The first main result asserts that if $G$ is a graph with ${\gamma }_{\mathrm{cg}}(G)\ge 3$ and $x\in V(G)$, then ${\gamma }_{\mathrm{cg}}(G|x)\le 2{\gamma }_{\mathrm{cg}}(G)-3$ and the bound is sharp. The second main theorem states that if $G$ is a graph with $n(G)\ge 2$ and $x\in V(G)$, then ${\gamma }_{\mathrm{cg}}(G|x)\ge \lceil \frac{1}{2}{\gamma }_{\mathrm{cg}}(G)\rceil$ and the bound is sharp. Graphs $G$ and their vertices $x$ for which ${\gamma }_{\mathrm{cg}}^{\prime }(G|x)=\infty$ holds are also characterized.