On the independence number in subcubic graphs

For a connected subcubic graph $G\neq K_1$ let $V_i(G) = \{v \in V(G) ~|~ d_G(v)=i\}$ for $1 \leq i \leq 3.$ Given $c_1, c_2, c_ 3 \in \mathbb{R}^+$ and $d \in \mathbb{R}$, we show several results of type $\alpha(G) \geq c_1|V_1(G)| + c_2|V_2(G)| + c_3|V_3(G)| - d.$ We also derive classes of graphs $G$ showing sharpness of these lower bounds on the independence number $\alpha(G)$ of $G$.