where the plus sign corresponds to the stable branch and the minus sign to the unstable branch. There exists a value of a critical value R e c r ′ where the above two roots are equal ( σ σ r = − − | l r | / 8 β β r ) such that R e c r ′ < R e c r , indicating that the flow in the region R e c r ′ < R e < R e c r is metastable, that is to say, in the metastable region, the flow is stable to infinitesimal perturbations, but not to finite amplitude perturbations.