On asymptotic values for the minimum number of spanning forests in simple regular graphs

Let $F(G)$ be the number of spanning forests in a graph $G$ and $\mathcal{C}(n,d)$ be the set of all connected $d$-regular simple graphs of order $n$. Define ${\widehat{f}}_d={\mathrm{lim\hspace{0.17em}inf}}_{n\to \mathrm{\infty }}\{F(G{)}^{1\mathrm{/}n}:G\in \mathcal{C}(n,d)\}$. Let $n_i$ be the number of vertices of degree $i$ in $G$. In this paper we give two lower bounds for $F(G)$ in terms of $n_i$ in connected graphs whose vertex degrees belong to $\{2,3\}$ and $\{2,3,4\}$, respectively. Furthermore, we determine the exact values of ${\widehat{f}}_3$ and ${\widehat{f}}_4$.