Solution
= Solution
For $t>0$, $\dot a=nt^{n-1}>0$ and $\ddot a=n(n-1)t^{n-2}$. Consequently \b[inflation occurs exactly when $\boxed{n>1}$.] The case $n=1$ is coasting, and $0<n<1$ is decelerating. The <Hubble slow-roll parameter> is $\epsilon_H=-\dot H/H^2=1/n$, giving the same acceleration criterion $\epsilon_H<1$.