The evolution of the actual disturbance is given by the real part of A ( t ) i.e., by | A | cos ⁡ ⁡ ϕ ϕ . Here the real part of the growth rate is taken to be positive, i.e., σ σ r > 0 because otherwise the system is stable in the linear sense, that is to say, for infinitesimal disturbances ( | A | is a small number), the nonlinear term in the above equation is negligible in comparison to the other two terms in which case the amplitude grows in time only if σ σ r > 0 . The Landau constant is also taken to be positive, l r > 0 because otherwise the amplitude will grow indefinitely (see below equations and the general solution in the next section). The Landau equation is the equation for the magnitude of the disturbance,