  Investigations on the resonance in the collisional flavor instability (CFI)
of neutrinos, which were reported recently, are reported. We show that the
resonance occurs not only for the isotropy-preserving modes as pointed out in
the previous work but also for the isotropy-breaking modes and that it enhances
the growth rate of CFI by orders of magnitude. Employing the linear analysis
and nonlinear numerical simulations in the two-flavor scheme and under the
relaxation approximation for the collision term, we discuss the criterion for
the resonance, its effect on the nonlinear evolution as well as the influences
of homogeneity-breaking (k \ne 0) perturbations as well as of anisotropy in the
background on the resonance. We will also touch the cohabitation of the
resonance with the fast flavor conversion (FFC).
