Fig. 2

Left figure: The function $NI(N)$ against the constant 1 when $a(N)$ is linear. For $b=0.5$ we have considered $a(N)=0.5+N/8$, for $b=1.2$: $a(N)=0.4+N/100$ and for $b=8$: $a(N)=6+N/100$. Right figure: The function $NI(N)$ against the constant 1 with $b<0$ and $a(N)=1+N$. Here $V_R=1$; $V_F=2$.