Inflationary scalar Fourier mode
= Inflationary scalar Fourier mode
For nearly constant inflaton potential and constant $H_{\rm inf}$, the spatial <Fourier transform> obeys
$$
\ddot{\widehat\phi}_{\mathbf k}
+3H_{\rm inf}\dot{\widehat\phi}_{\mathbf k}
+\frac{c^2k^2}{a^2}\widehat\phi_{\mathbf k}=0.
$$