$\alpha$-attractor-type Double Inflation
  We study the background dynamics and primordial perturbation in
$\alpha$-attractor-type double inflation. The model is composed of two
minimally coupled scalar fields, where each of the fields has a potential of
the $\alpha$-attractor type. We find that the background trajectory in this
double inflation model is given by two almost straight lines connected with a
turn in the field space. With this simple background, we can classify the
property of the perturbations generated in this double inflation model just
depending on whether the turn occurs before the horizon exit for the mode
corresponding to the scale of interest or not. In the former case, the
resultant primordial curvature perturbation coincides with the one obtained by
the single-field $\alpha$-attractor. On the other hand, in the latter case, it
is affected by the multifield effects, like the mixing with the isocurvature
perturbation and the change of the Hubble expansion rate at the horizon exit.
We obtain the approximated analytic solutions for the background, with which we
can calculate the primordial curvature perturbation by the $\delta N$ formalism
analytically, even when the multifield effects are significant. We also impose
observational constraints on the model parameters and initial values of the
fields in this double inflation model based on the Planck result.
