  This paper considers warm inflation as an interesting application of
multi-field inflation. Delta-N formalism is used for the calculation of the
evolution of the curvature perturbations during warm inflation. Although the
perturbations considered in this paper are decaying after the horizon exit, the
corrections to the curvature perturbations sourced by these perturbations can
remain and dominate the curvature perturbations at large scales. In addition to
the typical evolution of the curvature perturbations, inhomogeneous diffusion
rate is considered for warm inflation, which may lead to significant
non-Gaussianity of the spectrum.
