= Solution
When $V$ is approximately constant, $dV/d\phi\simeq0$, and exponential expansion gives the constant <Hubble parameter> $H=H_{\rm inf}$. Take the spatial <Fourier transform>
$$
\phi(\mathbf x,t)
=\int\frac{d^3k}{(2\pi)^3}
\widehat\phi_{\mathbf k}(t)e^{i\mathbf k\cdot\mathbf x}.
$$
Since
$$
\nabla^2e^{i\mathbf k\cdot\mathbf x}
=-k^2e^{i\mathbf k\cdot\mathbf x},
$$
each independent <inflationary scalar Fourier mode> satisfies
$$
\boxed{
\ddot{\widehat\phi}_{\mathbf k}
+3H_{\rm inf}\dot{\widehat\phi}_{\mathbf k}
+\frac{c^2k^2}{a^2}\widehat\phi_{\mathbf k}=0
}.
$$
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