On the Decision Number of Graphs

Let $G$ be a graph. A good function is a function $f:V(G)\rightarrow {-1,1}$,satisfying$f(N(v))\geq 1$,foreach$v\in V(G)$,where$ N(v)={u\in V(G), |, uv\in E(G) } $and$f(S) = \sum_{u\in S} f(u)$ for every $S\subseteq V(G)$. For every cubic graph $G$ of order $n,$ we prove that $\gamma (G)\le \frac{5n}{7}$ and show that this inequality is sharp. A function $f:V(G)\to \{-1,1\}$ is called a nice function, if $f(N[v])\le 1$, for each $v\in V(G)$, where $N[v]=\{v\}\cup N(v)$. Define $\bar{\beta }(G)=max\{f(V(G))\}$, where $f$ is a nice function for $G$. We show that $\bar{\beta }(G)\ge -\frac{3n}{7}$ for every cubic graph $G$ of order $n$, which improves the best known bound $-\frac{n}{2}$.