Non-perturbative approach for curvature perturbations in
  stochastic-$\delta N$ formalism
  In our previous paper, we have proposed a new algorithm to calculate the
power spectrum of the curvature perturbations generated in inflationary
universe with use of the stochastic approach. Since this algorithm does not
need the perturbative expansion with respect to the inflaton fields on
super-horizon scale, it works even in highly stochastic cases. For example,
when the curvature perturbations are very large or the non-Gaussianities of the
curvature perturbations are sizable, the perturbative expansion may break down
but our algorithm enables to calculate the curvature perturbations. We apply it
to two well-known inflation models, chaotic and hybrid inflation, in this
paper. Especially for hybrid inflation, while the potential is very flat around
the critical point and the standard perturbative computation is problematic, we
successfully calculate the curvature perturbations.
